You're better off either purchasing a rifle you can CONFIRM is preban (by SN), or go with a post-pan. You can also get some brakes that look almost exactly like the A2 brake. If you don't NEED a collapsible stock, postban ain't bad....
By law, even if a lower was produced before the ban, if it wasn't assembled into a rifle before the ban it is a post-ban. It's the date it was assembled, not the date the lower was produced.