For the 7.62x51 NATO to be used in an M1A, why would you use anything other than US GI stripper clips? I don't know just how available they are now, but just a few years ago they were plentiful. Many cartridges have the same rim size, i.e. .30-06, .308, even .45ACP, but all cartridges don't behave the same when trying to strip cartridges into an action of a rifle or a magazine.
I'm sure if you just check a few surplus dealers at gun shows, you will find the GI variety.