Only two OEM PTO caps were used, all others are aftermarkets. The first 9N cap was cast iron and shorter. The 2N introduced the steel version, longer, and used thru the NAA. When the 1-3/8 PTO splined shaft was introduced, it was a tad longer than the original hence they offered a different cap -this is the one with the squared off end. Was your shaft changed and you are using the OEM cap?
NOTE: The 9N-726 cap is cast iron; the 2N-726 cap is stamped steel. Both have identical Thread Size of 2-5/8"-12 UN 2A.