You say 1938 but not s/n - there were just under 900 styled WCs late in the 1938 model year
The generally accepted changeover point was late 1938 beginning 74330. Check your S/N as well as engine S/N and post it here. I guess its possible that an earlier tractor may have been set aside due to a problem and completed with new sheetmetal, but I would think it would have to be close to the changeover point.
Other possibility is dealer changed over if it was obsoleted inventory or if buyer wanted new style - I know Case dealers repainted a few earlier orange tractors to the 2-tone gold/orange to make them more appealing - typical buyer might not want to pay full price for last years tractor.
Or could be a user modification.