Dave,
thank you. It clarifies the changes. I checked the spare parts lists. The 1937 model with the 6P/ engine has the symbol F, as Doug stated. The model F has only a few different parts. If the plug became larger (shown as 18532 Plug, grease gun nipple?) they had to change the gearcase and the washer, as stated. The other change is regarding the layshaft and its ball bearing (H/101).
The 1938 parts list shows the gearbox parts of model F only without changes.
My gearbox has the plug marked by the word oil. Doug thinks, it is a 1938 model due to the gearbox number (6Q/202). Eric, Aero owners know him, has a gearbox marked with 6Q/237. May be ....
What I don't know is, where is the possibility to drain the oil out of the box. It is mentioned in the Pitman book. I have only the inspection plug, which is above the minimum oil level.