The romantic in me sees it as a ladies button carried by a soldier who wished to always have something of his loves with him.
And, I hope it was dropped as he scurried to safety on the battlefield and they were able to love a happy and long life together after the war.
(Once cleaned, try a graphite and paper rub on the reverse and you might can get part of the maker's info to learn more on the specifics...yes, I'm one of those "button people." I've loved buttons since childhood and fondly remember the old tins which smelled of mothballs and could spend the day going through the buttons putting like ones together- first on giant safety pins and later with needle and thread. It's fun creating the garment each button could have come from and the reason the buttons were kept when the garment was put in the rag bag. Yep, I think I was an easily entertained child.)