re: I have a question about this lower (Cal = Multi)?

To be honest, 99 percent of the time lowers are lowers, main difference being forged vs. billet. Some companies may have a nicer finish or tighter tolerances, but as long as you're mil spec you are good to go.

Just read reviews and avoid any companies with bad reputations (R-guns come to mind). I use PSA lowers (paired with BCM uppers) for my builds and haven't been disappointed.

I haven't been perusing the black gun forums in a while but I believe there are really only a handful on manufacturers that produce the raw forged lowers.

EDIT: This thread is old but explains my point about multiple companies using just a few manufacturers.