Now to the hook or hooked breech/breach. This image shows it better than I can explain with my rather inadequate grasp of English -

https://www.trackofthewolf.com/Categ...1/PLUG-FH-18-3

Basically, it is a means of quickly removing the barrel from the rest of the gun to enable thorough cleaning to be undertaken without endangering the lock or the woodwork. Often encountered on what the Americans call the 'plains rifle' - a usually heavy-barrelled and somewhat shorter than the usual long rifle that was handier to carry around on a horse - famed makers include the Hawken Bros of St Louis, but there were many others. The barrel is held in the stock by means of tenons and pins, either or both mixed. Simply drive them out, and carefully lift the barrel out of engagement.

Put it back in reverse, remembering to put the tenon pins or pins back the same way round - they are tapered to fit one way only.