Manual Steering box o-ring seals

Replacing the seals on my steering box. The later type with flat top. I have the larger o-ring for the rocker shaft but need the one for the input shaft. Can anyone provide the size for this (Rover 261855).

Many thanks
Have found the size. Went to the local Seal and Bearing supplier. The size for the steering input seal is 18.72mm id x 2.62mm cross-section. This is an AS568 #116 standard size o-ring. Also I had them size the o-ring for the rocker shaft and this was 29mm id x 4mm cross-section.