[Mondial]

Actually there is a model year encoded into the Zenith model number. Starting in 1936, the first of the 3 digits after the number of tubes gave the year. 1936 models had only two digits, the missing first digit implied zero years after 1936. So a typical 1936 model number would be 6-S-52. A 1937 model of the same radio would be 6-S-152, 1938 would be 6-S-252 and so on.

So the 15-U-273 would be a 1938 model. This numbering worked up into the 40's where a 12-S-471 would be 1940 and 6-G-601 would be '42.