You're correct on how the lower frame of the 1911 just has grips that get screwed in from the sides. The 2011/hi-capa frames are seperate from the grip, the entire grip being made of plastic/polymer.
the guns are definetly related, some parts are interchangable suchs at the triggers, slides, and they retain that similar look. The main difference being that the 1911 is a single stack mag while the 2011 is a double stack mag. I'm sure there are people who know more on this topic, i'm just putting in what i know so far.
hi-capa grip:
capa lower frame
capa slide
where on a 1911 its simply this instead:
