Particularly with the larger winding rotor, I get the feeling that some of these changes might be driven by the desire to use the BVL138 as a tractor movement for complications. With the perpetual calendar launched and more complications likely, it is possible that Bulgari has adapted all versions of the BVL138 whether they drive secondary complications or not in order to standardize production. The alterned back side of the movement might reflect changes made primarily with the dial-side interface in mind.