Some of the older bolt bodies had a tenon on the inside and the bolt head would have a corresponding groove that the tenon would fit into. This was done so the monkeys at the factory wouldn't accidentally install the bolt-heads 180 degrees out (upside down). They have since trained their monkeys better and thus eliminated the tenon/groove fitment.

Sooo....basically the fix is to take your handy-dandy multi-purpose Savage Whatchamacallit Tool (aka a Dremel) and grind out the tenon on the inside of the bolt body so the new bolt head can slide in.