Type of washer plumbing advice

Type of washer Hi Team,

I have a leaky radiator as it washer must of got damaged when it was fitted

Anyone know what this washer is called? And where I can find a pack?
It has a chamfer or bevel to it

Outer diameter 23mm
Inner diameter 20mm

Please respond if you know the answer to this question rather than alternative responses


Thanks in advance

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