Bill - Here is what I found through
GM Partswiki for Gear - flywheel ring GM# 3835306 in Chevrolet and GMC Master Parts Books:
Chevrolet Truck P&A 31Chevrolet Truck P&A 31SGMC X-164001The 1st reference says "1938-48 ALL", which would include all the 6 volt flywheels over those years.
The 2nd reference says "1938-54 All (exc. A.T.), again including those 6 volt manual transmission flywheels.
The 3rd reference says Series "Y" (1955 GMC) for 248-270 engines w/mechanical transmissions.
I think this is your confirmation of your 3rd hand info being accurate.
