This might have been stated already, but the Lemat has a separate shotgun barrel underneath the rifled barrel and this barrel is not rifled. It's a moot point as was already stated because muzzleloaders are exempt from this silly law. I used to have a Pietta reproduction of the Lemat and shot it a few times, neat pistol, wish I still had it.
As for the gun in question, what would happen if somebody back in the 30's, prior to the law of course, rifled the last 1/2 inch or so? Even if it was just scratch rifling would this make the gun legal today?