Brian,
The screws are M6 thread, Jobo being a German product. Jobo supplies the plastic screws with red fiber washers which go under the screw head and rubber o-rings which are meant to sit in the countersink in the housing and seal the holes. I found that the o-rings could easily slip through the countersink though, and think something like a thin, soft rubber washer or some silicon would do the job better. My feeling is that Jobo supplies plastic screws to stop people from over tightening the screws and wrecking the fastening holes on the poorly designed plastic arms!

Jim