I suspect you poke a pokey thing in to the hole in the top plate above "A" and use it to disengage the latch at "B", but you do that *BEFORE* you remove the barrel. Currently everything is being held in place by that spring, and if you touch it at the moment, things may become airborne.
I get the feeling that re-fitting the barrel is going to be interesting.
There is a picture here showing how to push a pivot out... special punches are required.
You would be best advised to remove the hairspring to avoid damage, and don’t push the pivot too far!
I think the markings look fine compared to other photos on the interwebs, and there is no doubt about the case number. Also no doubt that PZ5123 is the listed part for this case.
Cousins has the old Sternkreuz R19 catalogue scanned on its site. Page 163 lists their Omega equivalents, but there is nothing in the 30.8 range, to help find an Omega part number: https://www.cousinsuk.com/document/category/watch-glasses
PZ5123 equivalent is listed as XAC 330.553 which are about £6-7.
There is a wealth of other information in this catalogue about sizing and fitting glasses which is useful.
If you are certain this needs a 30.8 crystal, then I think you will need to buy the closest match you can find to the installed one (see page 50 of the above).
The only other thing I can think is that you aren't measuring it right... what are you using for that?
I blame the idiot who created the zip file. In my defence I was sitting in the supermarket car park at the time waiting for my wife to emerge with the groceries. Apparently I'm not to be trusted with that task either.
Give me a few mins and I'll update the file with the correct two parts.
EDIT: Actually give me about an hour or so, my uplink speed has dropped into the glacially slow bracket. I suspect there are too may other self isolating people interwebbing at the moment.
I'll post a revised link once it has uploaded.