There were some built in 1975 but sold as 1976 models.  No different than now- the latest and greatest comes out in time for the fall farm shows.  There were I think a couple more things that changed during the run of late 66 series transitioning to the 86.  We have two original black stripe 1066s, SN 59175 and 61938.  The later one is only 110 tractors from the end of production.  I really should park them side by side and crawl around looking for anything different.