Mine might be different - it has two o-rings:
Both have a resting ID of ~80mm. The top one has a cross section between 3.25-3.30mm and the bottom one 3.10-3.20mm.
The channels they rest in have different diameters that I can't get the caliper into, approx 86mm. The rings are stretched to fit these, and the bottom channel has a slightly larger diameter than the top one.
If I was to speculate, I would guess that since the housing is imperial (10") that the o-rings are as well, perhaps 3 1/8" ID and 1/8" cross section.