First that is not a button. check this site.https://www.google.com/search?q=wwi...niv&sa=X&ei=E2ZWVaXNEoSaNumQgYgE&ved=0CCsQsAQ
I was told that the ones with numbers like yours, were worn by national guard units called up in WWII. I have no idea if that is the case or not. But if you don't find yours above, you will find one similar to it.
Right on with it as a collar disc, with threaded post broken off. I think earlier though, as it seems to match the first example here (with a different regiment number): Dating Metallic Insignia: Collar Disks