There are no part available for any of these guns. You have to get them made. the good news is that they hardly ever go wrong if properly set up and the 'Royal' is a beautifully built and very simple sidelock with very reliable ejectors. I'd avoid the VERY early models with one lock cocking on opening and the other on closing but they are very early and very rare.
More complex hammerless sidelocks can be tricky (see the post on the Lancaster with the broken spring). I do shoot thousands of rounds in very old guns and some do need rejointing from time to time but that is no biggie.
For reliability the 'Royal' is the benchmark. Except for hammer guns of quality without those pesky ejector things.
