It may be down to the frame mounts?
Mine go in horizontally through the spring.
Some other years, especially the bullet have the mounts vertically.
The seat brackets have a 90° bend at the bottom.
Hitchcocks list the seat with three part numbers.
SEAT, SINGLE FOR C5 (UNSPRUNG) 2014 on (Can not be fitted with pillion seat) 1770007/1990636/1990703