if you ARE wondering about the different types of lowers; further down QB's site's FF link at
POST#68 Rooster shows the 3 standard-metric-lower recoil shields. other lower variations would be selector markings and some have lightening cuts also but the recoil shield of the lower is what will or will not 'match' your receiver. not to say it won't FIT as any lower (including inch pattern) will fit any upper
none is really any better or worse for the recreational shooter IMO
i try to match my kit's lower when building but prefer the looks of the type 2