First of all, let me say that the Academy kit is a pretty good model. They just kind of skimped and mixed variants. For instance, the interior is for an M3A1 with turret floor storage boxes designed to go around the turret basket, so it's not so easy to just remove the basket to get an accurate M3 interior.
Secondly, both their M3 and M3A1 kits are virtually identical, however, there was a difference in the actual vehicles. For instance, the rear portion of the hull on an M3 is angular like the kit represents. The M3A1 had a rounded area at the rear portion of the hull, but the kits share the same upper hull.
Bottom line, they got an M3A1 interior for an M3 exterior making neither their M3 nor their M3A1 accurate. But they are still decent kits.