the pistol lowers are mostly an american thing . as you can have a ar15 in the states as a pistol, you CAN NOT put a stock on it in the states to keep it as a pistol, so you have the basic buffer tube that just hangs there , also the lower has to be labeled with PISTOL ONLY, up here ,if you have the pistol lower marked "pistol" it will be registered as a pistol, though we are allowed to have butt stocks on our pistols here, as it wouldnt change its classification anyway, they are a typical ar lower, theres no difference mechanically or physically between the 2 , just that the pistol one says "PISTOL" on it
at least that is how i have read it,
correct me if im wrong.