As to the WHY and “have the lower right” comments, those are some of the things you see when dealing with the large frame. There is no mil spec for large frame AR’s and companies wind up doing whatever they want. In many cases, they’re probably trying to NOT be an identical copy of someone else’s for legal reasons (but that’s just speculation on my part). The differences between all of the different manufacturers is why you’ll see it recommended to keep uppers and lowers from same manufacturer when possible to limit headaches. We have to be really careful that we watch the nomenclature when looking for parts or even reporting on our own stuff. The Armalite, DPMS, RRA was tough enough…throw in 30 other minor variations from other companies and watch heads spin.
Your lower is not wrong. It’s the style they went with. As for why they didn’t have the jig specifically for that one on R2B…don’t know.