Vintage internal o-ring for Girling calipers

I'm trying to rebuild my calipers for my 71 142 vs buying refurbished

I managed to get all my pistons out using compressed air, still have a couple of bleeders stuck that I'm working on.

But I'm thinking of taking the caliper halves apart so that it is easier to clean the cylinder surface.

There must be an o-ring inside, similar to this

https://www.bpnorthwest.com/caliper-o-ring-for-girling-caliper.html

But it was not included in my rebuild kit.

Has anyone sourced these?

Thanks for the link. The dimension looks the same as I found with the Girlings used on the 240. My hesitation about splitting the calipers has centered around the thought that the bolts holding the halves together are torque-to-yield. They didn't seem like off-the-shelf hardware to me back in the dark ages when I took these pics.

http://cleanflametrap.com/calipers/

P/N S7428 with VP-Autoparts ? It's the "Gasket Caliper between halves" for the late model Amazons with the 2 circuit Girling calipers up front.

They're a little more than $1/piece