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It has likely been faced a few times.

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A facing tool mills a small amount of material from the outer edge of the shell, the purpose to bring the faces into perfect parallel, prolonging bearing life.

When you subtract the material that was removed by the framebuilder when the bike was constructed, consequent facing when the bike was built up, and any other facings through the years...yes, it is possible to lose 2mm or so.

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One lesson for others from your experience; if the bb shell was faced properly once, there is no reason to ever do it again.

FWIW, I've got a '72 Raleigh International, the bb shell width measures 66.5mm. It was probably faced when it was new. With cup and cone bb's, the exact width isn't that critical-

I also expect it has Raleigh's proprietary bb threading so fitting most modern cartridge bb isn't an option anyway.

No, not this one. This is the International, just below the Professional in the hierarchy, full 531 frame and fork, Nervex Professional lugs, made at the Carlton factory, and Campy equipped. In fact, I've got the full '72 Campy Nuovo Record group on the bike, as the only thing the bike had that wasn't Campy originally were the Weinmann brakes, and the original owner swapped those out for Campy sidepulls when it was new. The bottom bracket is standard English thread-

There were several British bikes that came from the factory with BB shells measuring about 66 mm. Moultons commonly have a 66 mm BB and they were almost never faced.

Interesting.