The following points are offered as rather loose guidelines to  attempt to determine the time period during which a particular item was made.  These have been gathered from a variety of sources including a liberal dose of common sense.  Hopefully, you will find them helpful.

Examples of non-metallic insignia dating from prior to the late 1960s or early 1970s were made from natural materials & fibers, usually cotton twill, cotton thread or wool felt.  These materials usually feel rather soft to the touch & have a matte finish.  On the other hand, man-made materials & thread tend to have a harsher feel & a more “shiny” appearance.  Insignia made of cotton tend to become rather limp with  successive launderings, but those made of man-made fibers retain a certain amount of rigidity.  Another factor in this regard is the percentage of embroidery in the insignia - the more embroidery, the rigidity the insignia will retain with the passage of time.
If you suspect a cloth insignia claimed to be an original is, in fact, a reproduction, check the back to see if there are any loose threads in the embroidery - usually the case on all embroidered designs.  If they are present, carefully remove a loose thread or two & hold them near a heat source such as a match.  Natural fibers will burn evenly, leaving an ash.  On the other hand, man-made fibers are usually petroleum derivatives & will tend to “flare” & melt.  This is a useful ploy to use at a show or exhibition.  Should the dealer refuse to allow you to conduct this little test, beware of the claims of originality if the insignia is purported to date from prior to 1965 or so.  It is also a good rule to establish an ongoing purchasing pattern with a dealer or dealers with you have done business in the past & can trust.
If you plan to purchase items through a show or an exhibition, check with the organization holding the show.  If the show’s promoters are reputable, they will bar dealers known to have been caught misrepresenting their wares from the show.
Regarding metallic insignia, particularly those with any precious metals content, these items are usually hallmarked by their manufacturer.  There are references in print that identify these hallmarks.
Embroidered insignia made within the last several years often have a stitch around the outer edge like that use to finish a button hole, & there is often a “pigtail” of the stitch which beyond the actual insignia & is attached to the back with an adhesive of some sort.  An edge such as this is referred to as “merrowed.”  Regardless of the materials used in the insignia, this is a sure indicator of a fairly new item because embroidery machines capable of producing this merrowed edge came into use in the late 1960s.
Most machine-made Army, AAF & Marine unit insignia made from the 1940s through the mid-1950s were embroidered on an olive drab or khaki cotton twill material & the individual insignia were then cut from the bolt of twill.  On these insignia a hairline width of the OD/khaki backing should be visible around the outer edge of the insignia.  Unless the background color of the insignia was olive drab or khaki, it was completely embroidered.  An exception to this that one encounters frequently are those embroidered on wool felt backgrounds.
With regard to Marine divisional & corps-level, their wear was prohibited in late 1947.  To be genuine, examples of these items must have been manufactured prior to that date.  Certainly, examples of  them existed within the Navy supply system for many years after that.  These designs are still in use & are often applied to everything assigned to the division except the uniforms of those assigned to it.  Due to their manufacture prior to the end of 1947, they are genuine examples of these insignia, but one will frequently encounter reproductions of them for sale.
In 1957, the Army changed their uniform color from olive drab to “Army green.”  As a result, embroidered insignia made between 1957 & 1968, when the merrowed edge came into widespread use, should have a thin outline of Army green backing visible around its outer edge.
Navy rating badges have had the same general appearance since 1885, when the navy’s rating structure was codified & rating badge designs became official.  However, there are some tips that will them to be dated with some degree of accuracy.  Ratings have come into being & later abolished from time to time.  To use the rating badge of a first class petty officers blue uniform for the purposes of illustration, these consist of  an embroidered white eagle & rating device & three red chevrons.  Until the period immediately following the end of  World War II, the chevrons were appliqués - they were cut from red wool felt & stitched on the rating badge.  From about 1948, they were embroidered on the badge.  The “issue” blue uniform is of Navy blue melton wool, & the background of  the rating badge was of  the same material.  Private purchase “blue” uniforms were frequently of black wool gabardine, & rating badges were produced on the same background material.  Although since 1948, all navy rating badges are worn on the left arm, from time to time some ratings have been authorized to be worn on the right arm.  These were general among what were considered to part of a ship’s ordnance or deck departments, Boatswain’s Mates, Quartermasters, Gunner’s Mates, Fire Controlmen & the like.  But like all regulations, there are exceptions.  For example, Pharmacist’s Mates wore right arm rates in the early 1900s, & from the time of this rate’s establishment in 1898 until 1948, its rating device was a red cross.  After that date, the rating device was changed to a Cadesus.  Also, during World War II, several makers of rating badges hand-stitched the year of manufacture on the back in the area of one of the chevrons.  Sometimes the maker was also identified, such as NYEC, denoting the New York Emblem Company.  I have generally encountered fewer examples of fakes or misrepresentations among navy rating badges than other forms of  American insignia, but they do exist as these items have become more highly collectable.  The worst example I have encountered was at a recent show at which a dealer had several Naval Aviation Pilot (enlisted aviator) rating badges for sale.  Due to their age (pre-1949) & as with all things concerned with aviation, these are prized by collectors.  This dealer had several examples that were complete with 1943 or 1944 dates on the backs, but they were outright fakes.  The original rating device had been removed & the aviator wings added in place of the original.  The giveaway was that the new embroidery was sealed with something resembling epoxy!  Again, the best advice is to examine each item carefully & to try to do business with reputable dealers.
Frequently in the case of  the insignia of  numerically smaller units, such as an aviation squadron for example, these were often made by hand.  Even on a machine-made example, often only the actual emblem is embroidered. leaving the background visible providing it is in the appropriate color.  Some examples from World War II & before were hand painted on leather or were made by cutting out pieces in the appropriate color & hand stitching them on to the background.  Again, unfortunately it is rather easy to produce a quite authentic appearing fake of these leather items.
On old original examples of cloth insignia that were not fully embroidered, a cotton gauze or cheese cloth backing was applied to protect the stitching.  This is most often present on examples done on felt, but it is also present on cotton examples.
Hopefully these items will prove helpful to you.  If anyone has any additional tips to pass along, let us know, & we will include them in later editions of our catalogs.  Perhaps the best advice is to do business with those who guarantee their merchandise.  Remember, it is your money, so spend it wisely.

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