I had an HD-10 with the 1939 serial #. Like yours it was at the tale end of the 1939 serial # allocation: 315167 to 355971. I always assumed they got an early jump on the 1940 models for Christmas that year. Since the Great Depression was starting to lift by then, companies were going all-out to see profits again!
That or some post-merger Evinrude employee got into the mix – Lord knows back in the 20s and 30s they didn’t seem to know the first thing about orderly model and serial numbers… 😛