How to soften a clamping screw

I'm cleaning up a spare Deckel high-speed head and matching 16mm collets to prepare them for sale. The motor for is mounted to a dovetail bracket that slides on a dovetail mount. (The motion lets one loosen/tighten the drive belt.) The bracket is clamped to the dovetail with a steel ball-handle-and-screw. On mine, the steel screw was clamping directly on the dovetail. I don't think this is what the designers intended: it has left some minor galling on the dovetail.

The parts diagram (FVV2)shows a "Bolzen" in between the screw and the dovetail. I'm not sure, but suspect it's a brass plug, with one end tapered to match the profile of the dovetail, and the other end squared off for the screw to bear on.

I am going to turn a suitable brass plug and taper it with a file, to bear against the dovetail and prevent it from galling from the steel screw. But before I do that, could someone tell me, is there a trick to keep this plug from falling out of the hole when the bracket is pulled off the dovetail? It seems like a standard design problem, and I hope there is a better solution than "stick it in with a lump of grease as glue".

The hole is threaded right through to the end,the diameter is slightly decreased where the brass plug projects through. But this is not enough to keep a brass plug in place.

Cheers,
Bruce