Hi
I bought item A263, I think it's the only cannon marked as sold
The 1939 rule is odd
All pre-WW2 muzzle loaders count as obsolete caliber and can be kept as "curiosity or ornament". The obsolete calibre list also includes things like 10 bore shotguns and pistols in .44 Russian so it's almost generous.
A smooth bore cannon over 24" long is a shotgun. I can lend my pre-1939 ornamental cannon to anyone with a shotgun certificate for up to 72 hours and they can shoot it.
I have a shotgun certificate, so I can borrow it myself. I could put it on the certificate, but then the police could tell me how and where I could keep it
best regards
Squire Robin