I've had a 1980s plastic Bluemels 3 for years, but recently got a BritanniAlloy Grand Prix for my 1953 Holdsworth.
Uses leather cup washers which were well knackered.
(I removed the multicolour rings with a hot air gun)
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Popped in to Belton and Slade - Wandsworth high street
https://www.beltonandslade.com/
They had an old cardboard box in the back full of leather cup washers - £1 each.
If you're going past it's a damn fine old fashioned shop.
Fixed for £1 and I splurged on a spare too.