gjberche,
It has been awhile since I have goofed around down in that area, so this might be bum info.
I thought the distributor was held into the block with a 5/16" bolt in a elongated hole to allow for timing adjustment. That bolt goes through the bracket you are speaking of, which is clamped around the housing with the funny screw.
The vacuum advance unit is also attached here, and maybe a octane selector if the rig is old enough.
That screw isn't made to loosen for distributor adjustment. It can be installed to the proper depth and tightened while the distributor is out...easy access on the bench with a screwdriver. I thought some of those had a small hex head as well as the screw slot.
Stuart
