Nah! That's not an electrical box related anything. It looks to be standard pipe threads but larger diameter than ordinary bases for the old galvanized pipe railing -- more like 3" even ? The guys suggesting an umbrella mount are probably right. In the days before ROPS were standard, one would often see a large heavy duty umbrella mounted on a single post. I vote for that guess. Never noticed how they were mounted.
Studying the picture, that threaded mounting hole is offset from center of the baseplate. Very odd/curious as to why. Makes me suspect whatever it is was repurposed from some other unrelated application and became handy for a mount on the fender.