I dont think Ive ever seen the Calvary crossed sabres and the Artillery bomb together. It reminds me of French Marine Artillery with the bomb on top.
http://collectionboutonsempire.e-monsite.com/album-cat-1-97188.html
Many iron backed buttons are civilian but it may be TIB.
http://www.vintagebuttons.net/shanks.html
Here are various tinned iron back buttons (TIB), some are uniform and some are political types. This is an older metal button back (antique era pre-1918 est.). Japanning (paint) was also used to stop the cheaply made thin iron backs from rusting although many buttons are found without the paint which was fired on but did not adhere well to the button back. Many "victorian" type fancy buttons have this type of back.
that's what I thought at first was french, but it was dug in my front yard in asheboro, nc @ 5 or so inches deep. could it have been a ww1 or ww2 souvenier brought back and lost later on by a kid playing war? i dunno. just a thought.
Backs of work clothing or overall buttons c. 1910-1930. Some had japanned (painted) backs originally but it's usually worn off. Cheaply made tin backs some with "floppy" thin wire shanks and others with a "riveted" (stud type) back. Those types were permanently affixed to clothing (usually overalls).