although palmers book states 40
i have had 3 39 tanny's and have confirmed with 2 others that had 2 and 1 39 tranny's
all 6 tranny's had the vent on the case and all but 1 were 39 only geared tranny's the last was a 3&r trans
I am with scoot - I think late 1939 and it was announced in the 1940 Literature. I have seen a couple myself.
A few years back I restored an original 1939 EL that was very high serial numbered and still owned by the original purchasers family - although it had received a California paint job sometime in the 60's - candy apple. At the time I thought it was possibly made on the last day of 1939 production. I have since forgotten the serial number but this bike had the vent and 2 other pecularities. One of the hubs was a step hub and the other hub was what is commonly referred to as a 1940 to 1944 hub and the oil tank was of the 1940 and newer style and I could not prove that it had ever been any other way.
The motor was standard bore and the speedo read less than 20,000 miles.