I believe this to be US military but what is it?

[brock]

I know its a box. But what was it for?

[bodes]

Maybe for a battery of sorts?...Not the kind for a vehicle, but for some sort of piece of equipment....Looks to be made to be waterproof....Bodes

[MikeP]

Reminds me of a case for an obscure short range commo device we kicked around in SF Training Group in the 60s.
Can't get enough memory cells to kick in to say more.