I saw lots of replies to your post, but I don’t believe the answer is there...
If I recall correctly, the original vaccine and the second vaccine were different - one was dead virus and the other was live virus. Whichever was first was deemed less effective than they thought, so recommended the second.
I could be wrong, or my memory could be faulty (It’s happened...), but I think that was it.
I believe that was right —the 1960’s shot was dead virus. But once I HAD the measles, and boy did I, I am not sure why I needed a vaccine.