Looks more like an LM style 5/16" x 24 than any type of NMO mount. The slot at the top of the stud is a dead giveaway, it's used to keep the assembly from twisting as the plastic retainer is tightened. I have one of these, and the whip attaches directly to that stud. Chances are, if you spin that black plastic retainer off, the whole assembly will drop right through the hole. These mounts generally used a 3/8" hole, although there is a plastic spacer that allows them to fit in a standard 3/4" hole as well.
Either way, it's a piece of junk and the best you can do is remove it, check that the hole is 3/4" otherwise drill it out, then buy yourself a proper Motorola/NMO mount with new coax attached, and a decent antenna to go along with it.
