There would have been a restriction plate fitted originally, but not one now. There was evidence of a place it could have been top face fitted and not recessed. The task now was to create a slot below the top face so we could glass in this brass plate to act as a stop for the arm of the centre plate.
Having now got the slot cleaned and down to a flat surface ready to glass in. The next picture shows the plate glassed in and just slightly proud of the underside of the casing top board. To get this flush we had to produce an indent in the underside of the top board so the restricting plate sat into it.
So all done and the final picture shows the surface mounted plate fitted on the casing top board to correspond with the restriction plate.