I have made one such pistol and it didn't turn out as well as I hoped.
As Jim said, some time you can bend the spurs out to facilitate letting in the cap. Be careful. I ended up with brass castings that were just too brittle, regardless of how much I annealed, and had to deal with broken spurs and then trying to let in a new replacement butt cap casting into a mortise cut for a broken and unrepairable part.
I ended up tracing the spur profile on each side where I wanted it to go. I filed a draft on the spur and cleaned up the pin on the end of each spur that goes into a hole drilled into the stock that keeps the end of the spur in the wood. Drill that hole at a downward angle so it draws the spur down into the stock. The outlines of the spur mortice were made narrower than the finished part and widened with scrapers as the part was let in. I then used the cut-a-little-and-try method to slowly move the butt cap onto the butt from the back, sliding up the curved spur mortices. The butt largely fills the interior of the butt cap - it is not cut flat. As Jim said, long and tedious process!
I hope this helps.
Best Regards,
John Cholin