I think a combination of storage medium and date would lead one to a relatively educated conclusion.
I believe boxer primers didn't come until late 70s/early 80s. The spam can doesn't necessarily denote corrosive, though in conjunction with the date (1973), I would bet dollars to donuts that it is.
Some later calibers and rounds come in spam cans - ie. the 2020 Norinco .308 rounds are in a spam can and they are not corrosive.
Anything from the early 70s and earlier for 7.62x39 in spam cans I would treat as corrosive as they almost certainly are.