Hello Stephen,
In my experience nearly all vintage gear pimps will boost the price if they can make the smallest connection to any famous, studio/artist, etc. that is provable.
The Sonic Circuis guys are pretty loose with their historical facts. About 12 years ago I supplied them with the U73 adjusting manual and the information to open up the low end, they then marketed it as "EMI mod" for the U73.
There were probably a few mics that were bought from Indigo at an EMI sale, those sales were quite normal in the 80s, but the ones I saw from EMI Colonge had a stock/inventory sticker on the bottom of the PSU and as far as I know the rest of the EMI studios used the same or a similar sticker.
There is still a small chance that it is an EMI mic, you might want to contact the Beatles Recording Book guy.
Best regards,