Gurley Transit Manufacture Date

I have in my possession (note that I didn't say "own"; it's a long story, the end of which is yet to play out) a 20" Gurley Hell Gate transit with an optical plummet. The S/N starts with TE, followed by a 4-digit number. According to How Old Is My Gurley, the code used is as follows:

W A N T G U R L E Y
0 1 2 3 4 5 6 7 8 9

That would put the year of manufacture at 1938. However, Gurley didn't patent its optical plummet until 1960, and the gun certainly doesn't look like it was made in 1938. Is there an alternate coding scheme out there somewhere?