Tug,
Good question.. These are what we Yanks call DUI's or Distinctive Unit Insignia, or better known as Unit Crests. These are accoutrements that are worn on dress uniforms in the US Army. The large majority of Army units have a DUI that is special to their unit's lineage and history. The DUI's can be traced back to the 1920's and in some cases earlier. You will see many photos of the various DUI's being worn on the lapel of a uniform up until WW2 and then they moved to the shoulder's. In most cases these were issued at the unit and when you left that unit and went to another unit you kept the outgoing unit crests and then replaced them with the new crests of the gaining unit.
These are another area of collecting that is very popular in the US. There are several thousands of these DUI's and most branches of the Army have them, Signal, Infantry, Armor, Military Intelligence, Artillery, etc.
Some collectors specialize in a specific branch and look for all the variations. Foreign Made US DUIs are very sought after and some are quite rare and can bring a lot of money. A few DUI's that come to mind are the Nuremburg War Trials DUI's which are miniature examples of the larger Nuremburg Shoulder Patch. These were made in post war Germany and again are very hard to find.
The earlier DUIs were screw back devices that went through the pre made holes in the collars of Army 4 pocket uniforms, then they went to pin back and then ultimately the clutch back system that is the 2 pin system with clutches.
I have a large collection of various branches because when I started collecting them they were very cheap. I have several thousand now as well but I am now just focusing on Infantry DUI's as that was my branch when I served in the Army.
Unit crest collecting is fun and you can gather quite a few in a very short time.
Smitty