Date codes consistent within roll for slides processed by Kodak?

I am trying to organize thousands of my old slides from 1958 to 2000. I seem to have many orphans . . .maybe! For example, I have one set with date code NOV66F7 on the mounts. Then there are five NOV66F, with no 7. It's not just that the 7 is missing, as the spacing is different. But the ones without the 7 seem to fit in the roll with the 7, based upon frame numbers and subject matter. So the question is, do the codes relate to mount batch as opposed to processing batch? Am I to rely on the date (NOV66) only, and disregard the "F" or "F7"? I have many such quandaries, with images that seem to fit with others, except for the codes printed on the mounts.