Sam,
The conversion from a riveted keeper to a slotted keeper didn't happen in one step. In C1948 there was a "hybrid" Heiser sheath wherein the keeper was fastened by both slots
and a rivet. However, this rivet was not the same large copper rivet that had been used to secure the keeper since WWII. It was the smaller aluminum (?) rivet that Bo had previously used to strengthen the mouth of the sheath. Here is a pic that should clarify:
Best,