The photos below are of the grille spacer and screw removed from an unmolested 1970 MK II Cooper S.
The spacer measures 5/16" diameter by 1/4" high. The hole allows a 1/8" drill bit to be loosely inserted.
The spacer is made from a very hard black 'plastic'. The ends are flat and have a rough surface, as if they've been sawn or cut. The sides are slightly concave and have a shiny surface typical of extruded or moulded plastic products.
Contrary to previous suggestions the concave sides could not be as a result of the spacer being squashed when installed. The material is simply too hard for that to occur. In any case, drawings of the spacer in several parts books clearly show the concave surface. Whether this is by design or as a by-product of the manufacturing process is open to conjecture. Only the engineering drawing for this part can provide a definitive answer.
In the parts book the part number for the screw is PTZ604, which translates into
recessed pan-head tapping screw Type A 1/2" long zinc plated gauge 6. However I think it should be PTZ606 which is 3/4" long, the extra 1/4" to allow for the spacer. The screw in the photo is 3/4" long and is an original part. The screw is a firm fit inside the spacer so the screw and spacer remain attached to the grille when it's removed. If it were a loose fit the spacer and screw could be easily lost.
I hope this will assist someone to make authentic replacements.
... here's one I just prepared.... 
