There she is! Weird it's got two loudness tap wires coming off it when the schematic shows they're shorted together. Must be a goof.
There should be more lettering on it and a logo, It probably says 50K, 503, B50K, 50KB, 503B, or something like that and you're just missing it. Doesn't matter though, I can see that it's a style I've never seen before.
If there's no way to repair it then you could modify a Panasonic pot, since it looks like it's got the same bracket and and tabs, but the pins would all have to be rerouted with wire into the holes since the order is completely different.