That’s a good question. I can relate what I believe to be the answer but I wouldn’t bet my life on it because I could very well be wrong.
In 1942 a second cannelure was added to the Cal 30 M2 Tracer solely for identification. By the time the T10 was adopted the second cannelure had been dropped. There are exceptions, of course, such as the M25 bullet that I showed. I think it would take someone like Chris P. or HWS to explain it.
Early in the development of the T65 there were some '06 Ball bullets that had an additional cannelure added in order to crimp while still maintaining the specified OAL. That was only done for a short time. By 1946 Frankford was making bullets specifically for the 47mm case.
I also have some examples of bullets that were made for the 49mm case but had a cannelure added so they could be used in the early 51mm case.
But, I have never seen a U.S. 7.62x51 NATO tracer with a bullet with 2 cannelures. Obviously, I haven’t seen every one made and I’ve only pulled a few examples for examination. And I can’t speak for tracers made in any other country, although I know there are several.
Second cannelures on bullets such as tracers, AP, and API are not always in the same location which leads me to believe that they are there for identification only.
Maybe NATO Dave knows. He is certainly more knowledgeable on the 7.62x51 that I am. I will always cede to the real experts. I’d like to know the answers myself.