The Mag Mount was made by Progressive Sporting Goods, Inc.
6122 Middletown Road, P.O. Box 366, New Middletown, Ohio 44442-0366
They made a number of different scope mount models, but list 3 of this design for muzzle loaders. These are listed as the model MLMA (for 7/8" Octagon), model MLMB for (15/16" Octagon), MLMC (for 1" Octagon).
The mount is a 4 inch long piece of heavy duty aluminum with 4 Weaver cross slots on top that's designed to clamp on to the barrel flats by squeezing the mount tight with 3 strong allen head screws. There is also a hole in the top of the mount for an optional screw which can be obtained from the company to provide an additional attachment with the threaded screw hole from a gun's rear rifle sight once it's removed. I didn't request or use this screw.
I used the mount to attach a 1X pistol scope to the front end of my rifle barrel (a la "scout style") since this was the easiest 1X scope for me to obtain & the only legal magnification at the time. I attached my MLMA 7/8's mount to a 13/16" barrel by adding a thin layer of tape on my barrel and giving an extra tight turn of the screws and never encountered any movement whatsoever during it's temporary stay there using up to 90 grains Pyrodex P. It was a tight mount. The major drawback that I can recall was that with the smallish flats of my barrel, there wasn't quite enough of the side flats sticking up past the stock wood for the mount to maximize it's hold to without having to remove wood. The mount also may have been the equivalent to having rather high rings but it wasn't really an issue for me. I didn't want to do that for a temporary mount so again I chose scout style. The interior of the mount has a groove on both inside edges where it is designed to fit on to the flats of the barrel that are 3/8's inch deep, so your barrel should have that amount of it's flats higher than the stock wood to get a maximum holding surface. Again, this is where the optional top screw would provide additional attachment strength. It would be nice if you found this company to still be in business, they made a unique mount that absolutely fit my needs at the time. What brand of rifle are you trying to add a clamp mount too? The box has a picture of it mounted on what looks to be a TC rifle with plenty of room in the flats BTW.