If your Svea 123 123R, Optimus 8 8R , 80 Stove Cap / Filler seal has an old or lost O-Ring, here is the replacement.  If the O-Ring is gone, fuel can leak out or cause a fire where it isn't supposed to.  Yikes!  These cap seals fit the Non-R Svea models, as well.

These washers fit most of the small classic petrol/white gas stoves such as the Svea 123 & 123R, Optimus 8/8R, some Primus 71's, Optimus 80, 111, 99, 199, 323, and 535, etc.

O Ring Specifications:

Fractional Size - ID (inside diameter): 3/8" (9.8 mm)

OD (outside diameter): 7/10" (approximately the same as a dime, see picture) (17.8 mm)

Thickness - Fractional - 1/16" : Actual - 0.068"

The O-Ring is Viton Fluoroelastomer.