My family has owned a 1939 series 75 for many years. It never had a battery box cover. This never seemed strange since the battery is down on the frame. But,researching battery boxes for another post ,I found it should have one. Group 10.0127 part# 35005211. What does it look like?
Note: Box covers were normally used on batteries that were under a seat. The parts book continues to call for them in 38-39 when the batteries were moved to the frame. They were dropped in 1940 (Not needed?) except for the 4090 which kept the under seat battery.
Warren