I'm thinking they replaced the the lugs with what the hardware store had in stock. May have been a higher grade bolt than what was available in stock size. I have a few of these with a broken lug. The lugs were not bigger earlier. That style carrier is newer anyway. The older style was not raised by the lug but had a fender washer welded to the plate. Up to the mid sixties the lug nuts used a 23MM wrench instead of 21MM but thread and pitch were the same.
If I had to guess I would say this style tire carrier started with the 72 model year. The main carrier was lighter with stamped ribs for strength. Dug up a picture of my 2/73 FST. It has this style and painted pewter. The ones with the heavy plate with no ribs used the style with the welded fender washer for a spacer. The style you have was only used 72-76. The lug sizes was the same before, during and after this period. Why they were changed who knows but they were.