Please correct me if I'm wrong, but I understand that putting the +4 mag extension on the M1 is against federal law. Importation of shotguns with fixed magazines holding greater than 5 rounds is prohibited under the GCA amendments passed around 1989 (weapons can only be imported that have a "sporting use"); nor can imported weapons be converted to a form which would be illegal to import. See 18 USC 922 (l), 922 ®, 18 USC 925 (d)(3), and 27 CFR 478.39. This only applies to imported weapons, not US made ones. This law did not sunset with the AWB.
I am not an attorney, and I would love it if I am wrong, since I have a +4 sitting on a shelf! Can anybody show if I'm wrong?