Can I put a pre-ban collapsible buttstock on my RRA post-ban LEGP rifle? I was told as long as I find one made prior to 1994 I could do it.

You answered your own question.

The Buttstock is not the weapon, the lower reciever is classified as the weapon by the ATF.

If you have a post ban lower, the age of addition parts dosen't matter... you cannot (legally) put a collapsible stock on it.

To legally put a collapsible stock on an AR, you'd have to be able to prove that the reciever was manufactured prior to the ban date in 94' and that the reciever was either assembled into a complete rifle, or was in a parts kit that could have been built into a complete rifle prior to that ban date.

The LEGP doesn't even come near.... Any Rock River Arms reciever is post-ban and there for has a fixed stock.

well u can IF u have the shorty stock as the only allowed part remove the pistol grip, or
make the gun no longer restricted under the

crime bill ie
make the magazine non detechable, or make the gun no longer semi auto. with the last 2 options u can add whatever u want