That looks like a TRW/Speed Pro part number for a forged piston. Not sure of the compression ratio as I don't have a TRW or Speed Pro cross reference book handy but if you look it up there, you should be able to find out the exact specs.
From google, you'd never know that TRW ever made pistons!
It's a TRW part, I found an x-ref to Hastings, but TRW does not list any pistons -- for any make -- on their website, and searches 'speed pro' (and permutations) turns up nothing.
In fact, if I had to rely on the internet to find pistons for the AMC six, I'd be screwed.
I'm going to my local parts store and I'll see if they've got a catalog.