Neil it may be that whoever serviced this rifle found that turning the step towards the seal pushed it in too far when the cap was screwed on and it interfered with the gear timing, because the seal was already nearly flush with the end of the cylinder?
I'm guessing that on some rifles moving the piston rack forward by one whole tooth before putting the ratchet wheel in moved the rear seal quite some way back into the cylinder, making several spacer shims necessary? If you didn't have them handy it must have been tempting to get away without them?
I'm not sure the cap locking screw was really necessary, because the threads are so fine and the cap is very well screwed on anyway. I suppose over time the cap could unscrew itself by enough to affect the smooth running of the piston assembly though. A dab of Loctite necessary perhaps?