For anyone interested…and assuming that Bruce Hansen’s Erector Parts Guide (2011 Edition) is accurate, I’ve been able to pinpoint the date of my set to 1941-1942. Biggest giveaway is from the 12″ base plate (MN) in my set which has the Small Hole Pattern. Further confirmation of the 1941-42 date is from the 18 1/2″ Angle Girder (MB) which in my set is the black patina plated. Finally, the 3″ Angle Girder (MO) is either nickel plated or zinc plated steel (made between 1935-42)
My dad was born in 1937 so this also fits well with his story that he bought the set from an older kid.