They were combined with both, measurement and level of quality...at least that's what I was told some years ago, but I could be wrong. I do know that the highest number Dia Compe had the bearings in the pivots.

Ok, I may be wrong, I just checked and apparently the numbers are all about size, which i could have swore they also had something to do with the higher the number the better, not sure why I remember that being the case when it wasn't.