Older kits (kits that were tooled long ago, as long ago as the '60s) usually had an interior "tub" with the interior side panels molded all in one piece to the interior floor. These usually had very poor, shallow panel detail like armrests and window cranks, because the molded plastic tree that the interior tub was on had to be able to be pulled away from the mold itself. If there were protruding, 3-D details on the interior door panels, the interior tub would not have been to be separated from the steel mold. A more sophisticated way to mold interiors is to have the interior floor and side panels molded as separate pieces. That way, realistic 3-D door handles, arm rests, and window cranks can me molded into the side panels and still be able to be easily separated from the mold. Also, it's much easier for the builder to detail the side panels when they are separate from the interior floor. The separate side panel way of doing it was an improvement in kit engineering that came along years after the old style one piece tubs. Thats why some kits have one-piece interiors (usually kits that were tooled long ago) while others have the interior side panels separate (most newer kits).