Charles to answer you questions directly - Your seat's rod end has a chunk missing at the end!
There are two seat adjuster types, of which one has serrated axle (see EeVeeWee's picture) end and the other ends with a square. Consequently, the rollers are different.
To help better, please provide a picture of the other seat roller you got there - I believe I have a good idea what you have there, however there is a chance someone swapped the seats with a newer style and I would not want to lead you in a wrong direction.