Putting receiver in the case

[TARNATION !!!]

Simply put gents, you cannot replace the barrel set if the top lever is centered. The reason for that is the square bolt that engages the barrel lug to hold it tightly shut when in actual use has been allowed to slide into the lock position when pressing the trip down to center the top lever to store the gun. If you then try to close the barrels for reassembly the bottom of the lug hits the square bolt. Both those parts are very hard steel. You would have to break the bolt or the bottom of the barrel lug to somehow close the barrel set and that ain’t gonna happen even if a sledge hammer is used. (Well never tried that but I’m guessing that’s a correct statement).

For interest there are approximately 100 parts (plus or minus a few based on vintage) in a Parker Bros hammerless shotgun. Many are fixed in place and some move to facilitate operation.