Those are fabulous reference studies, my friends, and I thank you both very much for the information.
I am sure the gun was converted in North America, or at least by a person with only a particle of the skill of the Bonds. The drum was poorly made, and the removal of the pan and fence - a disaster. They filed away much of the engraving behind and under the pan, and I only guessed at "M" - it could very well have been a "W". All the holes where flint apparatus screws had been were filled with riveted steel.
The client is not particularly a student of firearms, and wanted me to make the fowler shootable, which it certainly is now.
I removed the drum from the patent breech plug, and found it to be .400" diameter x 20 tpi. I made the new liner 7/16"x 20 with a little countersink. The powder channel was less than 3/16" in diameter through from the bore to the back end, so I opened it up with a 5/16" end mill, and while I had it in the lathe, I gently recut the circular cone at the end of the threads, removing the darkened iron and polishing bright. The outside of the barrel has its original finish, while the inside had some rust and dark spots. However, it cleaned up admirably with just abrasive cloth on a drill turned rod, fed from the breech. It loads nicely, and more importantly, it cleans easily.
We didn't have a lot of time for pattern testing but the preliminary work shows much promise. At 25 yards, we are getting 84% patterns in a 30" circle, with 50 grains FFg GOEX, 1 "A" (1/8" hard card) wad, a gob of spit down the bore, 1/3 of a "C" (fibre) wad, the equivalent volume (3/4 oz.) of 7 1/2 shot, and one thin "B" over shot wad. It's very pleasant to shoot this little 24 gauge, and it'll be murder on grouse.
Thanks again for the insight into this darling's history.