You building one?
My old Drummond lathe is buggered so I'll not be finishing mine.
I can't remember everything as the parts and notes aren't to hand.
Old supersport cylinder, lugs were damaged. Its ideal, as the barrel locking pin is in the breech jaws, I.e no hole in the cylinder as its in the breech area under the barrel.
Turn the cylinder upside down, and back to front. Cut off the breech jaws. Weld the transfer port up.
You can now drill down into the cylinder near the "face" to replicate the transfer port.
The barrel will be an overlever type arrangement. The barrel will be build into the cocking arm. The cocking arm, bolted into a bracket that is welded to the cylinder near the old trigger area. A rail system was to be incorporated to mount a scope.
The trigger will be welded into the old "top" of the cylinder. Opposite to where it should of been. I'm sure the piston I had, had a slot for the trigger to engage, so cocking slot on the cylinder could be left on the top, so it can be cocked!
The transfer port area, would need to be custom made unless an example could be found. Ie from a webley nemesis etc.
The barrel would lock into the transfer port bracket around it.
I got no further as I'm on the hunt for a lathe, have been for a while
Not sure if it's what your looking for, but I was inspired to build one. I just haven't finished it.