Shimano QR o ring for seat post clamp

[tagnut69]

Does anyone know of a source of a suitable replacement for the o ring on an early shimano qr seatpost clamp. From the bits I have it appears to be 21x24x4.5 but this size or anything near does not exist.

 

[bikeworkshop]

Decent hardware store should have a box full - equally agricultural parts store - you ought to be able to get a good visual match.

 

[FZRIP]

O rings are specified by internal diameter and cross section.
There are sellers on eBay with 18/19/20/22mm id 4.5mm cs.
The size in your post doesn’t work for a round section ring.