Measurements for Omega f300 caseback gasket?

I've now assisted my second friend within a month in getting their first Omegas (YAY!). The most recent one is this f300, ref 198.020 and it is missing the caseback gasket. I have part number 088.0607 for the gasket but it's proving difficult to source. I'm curious if anybody has the dimensions for this gasket instead, inside diameter and cross section (ex. 30mm ID x 1mm) so I may source a generic piece? Thank you!

Yes, I know, pic or it didn't happen.

Ha! I think there may be a small clue. 😎
BTW I love this thing, have never had an f300 personally so they are now on my list after seeing that sweet second hand movement and that (to me) lovely whine it makes up close. Not to mention the applied logo and those chunky indices. Waiting to see if this one "sticks" with my friend or not, but if not, it's mine.

I'll have this in my possession until the gasket arrives, so will see if these really work for me too or not. I'm enthralled by the story behind the movement, but prefer an Omega to the Bulova iterations, so would be a fun addition.
I will add that my friend who picked this up has tinnitus, and I was worried a bit about his response, but he wasn't bothered at all to the whine. I will advise him not to keep on the bedside however.
Is the chap in the UK still the best bet for servicing these when the time comes? I read up on the potential faults before this was purchased and he seems like the man for the job.

I was ready to buy the black gasket that fits in the caseback channel but then realized there's also already a yellow gasket on the case side. When I searched images online for the 198.020 I can't find another with the yellow gasket, but they all have the black one on the caseback. It seems clear I should still install the black one, but is the yellow gasket correct, and should there be anything there at all?
Somewhat confusingly it feels like it is snugging down against the yellow gasket, rather than metal to metal as I would have expected without the black caseback one in place. If I add the black one would it be wrong since it might then double up gaskets, something that's generally avoided in most situations?

Also, I did ask the seller about the gasket and he said, "We installed a new yellow gasket in watch, its possible case back ring, got loose in shipping, we only snug on accutron movements, as you can damage them if too tight, just snug it up, and you should be good thanks."

Here's the full view of what I'm working with:


However, this seems more typical of what I'm finding with online pics, no gasket on the case side:

Thank you, I will remove it once the correct seal arrives and toss that yellow bit in the "may need this never" drawer.