Flatus Veteranus
I have a part copy of the bomb-bay tanks AP amended to Feb 68.
This is the console. Item 9 is the BB tank pressurisation switch.
The 'A' tank mod 527 included the fitting of pressurisation pipes. A removable pressurisation panel was located in the port bomb-bay. Unfortunately I do not have the section with more detailed descriptions.
Certainly the bomb-bay pressurisation switch was inoperative in my experience [70s] that backs up your conclusion on the incident you describe [1968ish].
The incident couldn't have been any earlier because 'A' tanks were fitted in the 301-engined aircraft of the Cottesmore Wing that were transferred to Waddo in the Spring of 68.
I understand the 'A' tank was the only bomb-bay tank compatible with Yellow Sun and Red Beard. Scampton Blue Steel a/c had 'As and Es' though the 'A' was fitted somewhat further back. Can you confirm that?