[Arran]

Mike;
Those button cells are carbon zinc cells that were produced, and marked by P.R Mallory, and were somewhat of an engineering "FAD" in the mid to late 1930s. Canadian Rogers, and Canadian Marconi built sets used a lot of them, typically two of them, one being on the wiper of the volume control. Personally I think it was a scheme for companies to avoid paying patent royalties, likely to RCA, though that would not have mattered in Canada as all companies belonged to a patent pool. Anyhow there are some modern watch or coin cells that are a viable substitute, except that you have to reverse the polarity of the holders because on the watch cells the exterior is positive and the center is negative, Apparently you can revive the originals by dropping the cell into boiling water, though I have not tried this.
Regards
Arran