"Steel" or iron mounts for London made Colt revolvers were cast malleable iron. I'd bet Manton the same. In my opinion you need a skilled TIG (a.k.a. GTAW) welder and a high nickel weld filler made for cast iron. That is, either about 50% nickel by weight, or about 90% nickel by weight, balance iron.
I'd have to read up on malleable iron metallurgy but I'd think that some manner of s-l-o-w anneal would be appropriate after welding.
Or you could find a guy skilled at silver brazing cast iron. This is not quite like like silver brazing/soldering steel, experience would be nice. Saves annealing, less chance of ruining it. I could dig up info on brazing cast iron if you wish.
Metallurgist for the last half century.
Jim K