It is a collar disc. The button is a WWI tunic button.
I have to disagree with the comment on the post war hat device only having the pin or nut come off. This is untrue. The US Service cap device has pretty much been the same 3 -4 part device since the Second War. Very little change has been made to it. Single piece plastic pieces were made during the war for austerity measures but were dropped after the war. The round backing plat in current pieces have a rounded underfolded edgw where the WWII pieces wereflat. The service cap was dropped from the enlisted uniform regulation in the late 70's but the enlisted dress blue hat still uses the same device but usually in a sta-bright finish.