Don't forget that Mullard was only a part of the Philips group.
If a 'Mullard' valve was in short supply or not a type made in the UK it had to have 'Foreign' or later 'Imported' on it to comply with BVA rules.
Philips coded (99%) of their valves with an etch code system so you can identify the valve type, the manufacture plant and date it was made.
Have a look round the bases to see if they have a Philips manufacture etched code.
If it is a PC88 there should be a 'JE' type code followed by the symbol of the plant it was made in and then the date code.
Not always easy to see the code though.
Lots more here:
https://frank.pocnet.net/other/Phili...CodeListAB.pdf
Like others I wonder if some that we see are now the genuine article. Bigger profit in labelling something Mullard...