The B64 was released before AE had their big supplier switchover with the B6.1. I'm willing to bet they didn't want to run a new batch just to restock the B64D. Especially since they had so many carpet kits still in stock and they'd have to deal with the B64 kits from one supplier and fresh B64D kits from another. Better they just they let the B64D stock run out rather than have to retool the line for the 74 6 months later.