The only real difference in the AMD hg retainer is the locater pins that fit *inside* the stamped sheet metal hg. They can be removed or matching holes can be carefully drilled in the replacement lower hg.
As for the legality of a bottom rail, virtually *every* new full size pistol being produced today comes with one from the factory as standard or optional equipment. All are just 5 seconds away from being an AOW by clamping on one of those plastic "broom handles" popular on AR15s these days. Someone brought up the issue of a multi-purpose rail versus the dedicated PG square mount hole on the original lower being different legally. If you really wanted to CYA while retaining the original hg simply weld a patch over the square hole, or drill it out larger.