I have two identical .50 BMG caliber rounds with the same headstamp, K 75 50, and silver tips. Knurled Crimping cannelures on the bullets, brass primer cup, what appears to be purple primer sealant and ring primer crimp in brass cases.

What's different about the cartridges is that one has a single knurled ID cannelure, while the other has twin knurled ID cannelures. What are the different cannelure types trying to convey?

I have another API silver tip .50 BMG round with the K 74 50 headstamp. It has only the single ID cannelure, all else identical to the 75 dates, so I'm assuming single ID cannelure was the standard up till sometime in 75 when the twin cannelures appeared.

I know the number if ID cannelures and primer sealant colors have significance in UK usage, so was wondering why the break in 1975 on cannelure types.

(Checked the Labbett series, no clues there.)