I just completed a PB on a Peter Niehart build. The spring is fed thru the buttplate. I feed the spring thru the plate while it is off the gun, then both are assembled onto the gun together. Having enough clearance everywhere is a challenge. File a bit there and there as you go. Mine was a top opening box. I loosely followed the great tutorial that Tom Curran has posted in the metal working tutorials. But as far as the sequence in which to install, I usually do my Patchboxes on the tail end of the build, so yes you will need the buttplate fitted, but I leave it off until the final stages of the PB install. Here is the sequence I used - Fab the box and finial then inlet. I then figure where the post will need to be integrated with the latch type being used. Install the post on lid. Fab & install spring latch into box(leaving extra length out the rear of BP) . Fine tune to get latch & lid working properly. At this point I plotted where the latch will need to come thru the BP. Make the mortise for the latch thru BP, trim the spring to length and braze on the button. Note that the mortise in the BP will need to be oversized to allow the latch to "snake" it's way into position, but I covered this with the brazed on button.
