The bumper is held in place by four (4) M8 bolts that are molded into the bumper. You can see the four holes in the picture below:
You access the nuts from the aft lazarette.
Unfortunately, you do not have access to the heads of the bolts since they are molded into the bumper. So if there is any corrosion on the nuts and bolt, the bolts will just end up spinning inside the bumper. At that point you will just have to pull the bumper off the bolt(s) and then re-seat them.
SM #440 Cara
Deale, MD USA