Rod - They were a standard BMC accessory - HYL1952 for mesh and HYL1953 for metal. Below is from the accessory catalogue.
I'll take some photos of mine later today and post them here. In the meantime below are some photos of NSW Police Minis.
Wish I'd known you are after one as I sold a spare period mesh one at the Grand Day Out last Sunday. They come up occasionally. If you want originality I would avoid the new ones. They have a reinforcement bar in the centre which the originals didn't have.
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BMC Accessories pS13.jpg
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TBL NSW BXQ 122 Cooper S.jpg
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nswmini.jpg