Sorry, I feel very unhappy about the description of the puzzle-bus.
Me, by myself, doesn't know about a bus manufacturer, named WUMAG. May be, I am wrong. A proof about, that teaches me better, would convince me about another truth.
All buses with WUMAG-bodywork I know about, are based on chassis' of common manufacturers of its time.
If the manufacturer isn't known by the puzzle-poster, I think it would me much more correct to change the headline of the puzzle to e.g.:
'Bus of unknown manufacturer with WUMAG bodywork, 1930', or something else like that