As fountain pens began to fall out of favor in the US, replaced by BPs, Parker introduced the felt tip pens. To accommodate this new type of writing instrument, a new section was made that could be fitted with the 75 line of pens.
The problem with the felt tip (FT) pens was that the ink was dry out rather quickly when left unused. Eventually this gave way to the rollerball (RB), for which a different section was needed.
It is quite evident from these pictures that the FT and RB sections are different at the writing tip. Here is a comparison of the openings with the RB shown on the left and the FT on the right.
It is for this reason that the FT can accommodate the plastic shim that allows the BP refill to be used.