Bingo! Note:

1. The numbers on the back of Heiser sheaths all the way through the 40's were length only. Model numbers didn't start until the turn of the decade.

2. From your photos, it "seems" that the keeper is not riveted! If this is the case, it is the first that I've ever seen (That also had throat rivets!). There was a "transition" Heiser sheath keeper circa 48-49 that had both a small throat rivet and slots (vs. the old large copper rivet), but the quantity made was really small. Here is an example: