Good job, f1fan, tracking this one down. Keep it up!
Here's the date of the drawing:
Although Tremulis had been drawing Duesenbergs for customers for their in-house "Walker" design team since 1933 (he styled some of the best: the 1934 Walker-LeGrande Model J Convertible/Coupes - J530, J534 and J535), I don't think he was hired as the Chief Stylist until December, 1936. So this may have been done during one of his stints at Briggs under John Tjaarda, another rear-engined fan and streamlining advocate. When I find the answer, I'll post it...
The 1935 date came from a photocopy that Tremulis had signed "1935" many years later as a gift to someone. Tremulis had stated previously that he relied upon this rendering as inspiration for the final design of the Tucker '48. Always ahead of his time, that little fixture on top of the bubbletop was a turn indicator up high so that everyone would be able to see your intentions.