with pre-ban and post-ban parts whats the difference

Depends on what state you live in.  It boils down to this.

If you live in NY/CT it means you are going to pay through the ass for a lower.
If you live in almost every other state, it means you can screw the people in NY/CT and make them pay through the ass.

Massachusetts enforces the 1994 AWB. Preban includes bayonet lug, threaded barrel (i.e. removable flash hider), suppressor, etc.  Postban can't have any of these.

ETA:  more detailed definition:

"A semiautomatic rifle was considered an illicit assault weapon, for example, if it featured a detachable magazine, as well as at least two of the following five attributes: a folding or telescopic stock; a conspicuous pistol grip; a bayonet mount; a flash suppressor or threaded barrel (i.e., a barrel that can accommodate a flash suppressor); or a grenade launcher."

If you live in CT, you can't purchase a pre-ban lower. You can only have one if you owned it before the ban and reigistered it thought the state police and have a cerificate, for possesion of a "assault weapon".  If you move there, you cannot have a pre-ban rifle either, you have to sell it to a dealer, sell it out of state or turn it in to the police.  Nice state....huh?

Been there......Done that.......

I think that is only with the colt guns registered as "assault weapons".