I had 2 different heads on my 273 as well. Also one of the rod bearings was dated 71 where the rest were 66. Something came loose along the way and was fixed.
Or they were leftover from years ago... Back in those days they didn't practice FIFO (First In - First Out)...
Those bearings could have been on the back of the shelf for years... Then if they were getting short of bearings and the shelves were getting bare, they grabbed what was left to keep the line running...