Here are 2 different 4 pin modules and they ground thru the hole on the right. Not sure if all ground like that but I cut open a 1970s AcDelco module and it only grounded thru the one bolt hole. I would put a screw in that empty hole on yours.
When you flip the HEI module over it has an aluminum pad and both those holes are swedged. I will check it with my Fluke VOM to see.
But I may put a copper strap between the two bolt holes as a "Just Because".
I could drill thru the steel of the OEM ECU and TIG weld a Stainless machine tapered head screw on and then fine grind the head on the front. Once its painted you'd never know.
IF I were building these to sell, that is what I'd do. The vendor "WELLS" that is making these for Autozone and NAPA should copy this mod for all the units they sell. I'd bet their failure return rate would drop.