One is stamped 1R and the other is 2R. Is 1R the top and 2R is the bottom? The #1 has a bigger free gap than 2 but they are the same thickness

Expanding the pic right up it looks like 1R is a 'Keystone Tapered' top surface ring. Regardless 1R is top groove but you 'should' find 2R would not fit fully into the top groove if you mistakenly tried it. Piston should have the letter K there somewhere.

I may be mistaken , but wouldn't an AT3 have used a Dykes top ring ?

 Ah, guess we need a pic of what piston he's using. Rainy day & nothing better to do so here's a google image of 316 rings partzilla says it takes. Pic coming. Looks like an expander under the second ring so OP check for the old one & might be stuck in with carbon if its never been apart. Expander isn't important item in my books, just my opinion & have seen them breakup but still retained.

Yes I forgot to add it's a NOS 316-11631-00-96 standard size piston and NOS 316-11610-00 standard rings. The bottom has the expander ring. The stock DT piston I removed had the thicker top ring but I'm switching to this piston and ring type because it was way cheaper and more available. The 74 dt125 shares the exact same cylinder part # as the older AT2-3 

1 should be top ring and 2 should be bottom ring.