I asked my Dad to contact their company's seal supplier.
They can supply this seal, that sized seal comes in 2 slightly different elastomer rubber types, signified by the colour. (apparently G usually refers to blue, as opposed to black). Phil or Fenton, just reconfirm what colour the seal is, Fent's pic looks black? These seals are now all rubber, rather than metal - rubber, but that won't make any difference. The supplier said they are on the shelf, cost approx. £3.00 they think.

Phil, I'm overseas later today for 2.5 weeks. You can call my Dad (John) directly at JP Plant Hire on 01977 683324, if you want him to get you some seals before I get back.
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Rich
MC21 Track Bike / RS250 NF5 'Spencer', NX5 'Cadalora' & NXA 'Aoyama' / RS500 / Two Brothers Racing RC30