Coordinates on aerial photo. Can you help?

My father, a mapmaker with the U.S. First Army, ETO, brought home several aerial photos, which I now have. Most of them are annotated with coordinates that use the French Lambert Zone system. I've been able to convert them to present-day coordinates using an online conversion tool. However, I am stumped by one aerial photo of a coastline. The labeling on the photo says: US 7GR/2346 17July44 F24" 20,000 27SQ.
This is all pretty clear except for "7GR" in the coordinates. I can find no information on a system that used anything like that in a coordinate. I assume "7GR/2346" is a truncated or partial coordinate. That is, there are larger grid coordinate prefixes for both latitude and longitude that are not stated.
I would appreciate any thoughts on how to interpret the "7GR" coordinate, or what type of coordinate system it might belong to.
The aerial photo, taken 17July1944, is of a coastal area, presumably 27 miles square, and likely somewhere along the coast of France's Brest Peninsula or possibly the Cherbourg Peninsula. It shows a causeway going out to a round island that appears roughly 1.5 miles diameter and is heavily built up with construction on the inland-facing side that appears to rise to a peak in the island's center. Perhaps a German observation post. I suspect this photo was part of a stereo-pair used in mapmaking and would have covered an area not yet taken by the Allies as of 17 July 1944. I am a new member here. Thanks in advance.

 

Can't help with the labels/coords, but that's almost certainly Mont Saint-Michel, W of Avranches.

 

Dunno what those mean, but on the Avranches-Fougeres 1:100,000 Sheet G6 that location is vT1611.

 

7th Photographic Group (Reconnaissance) Sortie/Mission 2346 - Flying F-5 out of RAF Mount Farm if it follows RAF PR image markings.

Ross

 

A few other 7th Group pictures and note there was some variation with the catalogue coding.

US Reconnaissance Photos Taken During World War II Revealed

 

Klambie got it right. Look at the shadow, that is unmistakably the Mont Saint Michel.

No frames, but a finding aid here:
https://ncap.org.uk/browse/finding-aids?keywords=US7/2346&daterange&bbox

 

Thanks immensely to each of you. Extremely helpful!