Harrison/Colchester index ring

I am looking for the index ring that sits on the topslide of a Harrison M300 / Colchester Student. Possibly other Harrison/
Colchester lathes may have the same size ring.
Harrison's part no. is 311 10 006
Anyone know of a lathe being broken or one being modified? I have tried the usual trade lathe breakers and 2nd hand tool dealers without success.
Many thanks in anticipation.
Dave
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