Gelatin hardened with alum can be surprisingly tough. The historic Woodburytype print process used a hardened gelatin relief to mould lead sheet by direct pressure. The lead could then be repeatedly inked or flooded with a gelatin/carbon mix and the image transferred to paper. Alternatively, a plaster cast could be made which reproduces the relief of the gelatin layer.
Direct carving of ceramics to create relief was the norm in earlier times, but was expensive and required skilled craftsmen, so was unlikely to have been used for door-to-door sales in the 30s. I suspect that a soft metal foil was burnished onto a gelatin relief, paint or other colourants flooded into the relevant areas, and then plaster or other ceramic cast into the resulting mould. Not a technique for large scale production, but for a one-off it would work well enough and would be simple and cheap.
There is a fair bit of information about Woodburytypes online, but this page in particular talks about some of the ceramic variants: