All above is correct. I collect these in autococker threads. The AC version can usually be had for around $35 shipped in excellent condition, at least that is what I have been paying for nice ones. Sometimes less on ebay. Fantastic barrels imho.
These come in a ton of different varients because you could pretty much order whatever you wanted at the time. Plated/unplated, rifled/smooth, and all sorts of really funky porting. Yours is actually not that funky. I prefer the zig-zag patterns.