In the 1920s Bulova had a practice of hand marking the case back with the last 3 digits of the case serial number. To make this task easier they used roman numeral, using an upside down V to indicate '0', as it was tricky to hand etch a circle. So in your case the last three digits of the case serial number are 400 or as you can see the hand engraved IIII /\ /\
When Bulova produced a case they needed to separate the back part from the middle or bezel block and in order to make sure the pieces fitted together properly later on and not get mixed up with other cases, they would quickly mark the back and/or bezel piece so they they could all be married together at final assemble to avoid the different case parts getting swapped and as such not fitting correctly.
This process made sure all the case parts belonged to one-another when assembled.