So...after checking the instruction manual for the bridge, it seems I am indeed missing the plug. I can't imagine I removed it & set it somewhere away from the bridge parts. Will call MTH & order a new one.
Have begun temporarily tacking track down to check fitment & position. Will then mark so I can lay down roadbed. The shelf boards are only attached w/a sg. screw at this point in case any movement is needed; the leveling issues should be improved if not fixed when all screws are in place.
Will keep you posted!
- Neal