It's a canteen token, used for pinball machines and to purchase refreshments from the facilities. If I understand the jargon correctly, "EOM" stands for "enlisted open mess".
There are hundreds of different types of US-Vietnam military canteen tokens, most of which are relatively common.
Here's an example from eBay, of the same token but without the hole, that sold for $7.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis