This is one of those times when words have more than one meaning.Why would they put any kind of seal on a full GI can of ammo?
Isn`t the latch and rubber seal around they lid supposed to hold through hell and high water.
Just my experience, but I don't know how many tens-of-thousands of 100-round 20 mm ammo cans I emptied and loaded aboard F-100s and F-4Es, but do know that I never had to break a tamper-proof seal, or saw a broken one. Unless the seals were removed at the ammo dump, to eliminate the possibility of flightline FOD, they didn't exist.