Only thing that I could see that comes close is SL which is
No. 13 Operational Training Unit RAF (13 OTU)13 OTU was formed in April 1940 as part of No. 6 Group RAF Bomber Command at RAF Bicester to train originally Bristol Blenheim light day bomber aircrew. It was disbanded when it became 236 OCU in 1947.
In the morning I see 51 in the afternoon I see it as SI or SL. The full photo shows a wet landscape which could still easily be Otmoor!
I have attached a preview download of the photo which shows the background as being marshy or flooded. The reason for posting it I wanted to identify the range as that target is not of a usual type, according to the manual. It is also very small, probably 16ft sides, which was still common early in the war but the open corners suggest a makeshift target.
The text with the photo is:-
"At a famous Air Armament School in Eastern England, bomb aimers who have proved themselves in operations are trained as "captains" of squadron bomb-aimers. Picture shows: bombing practice on the ranges from "zero" height during the Second World War."
It is dated 1st January 1944 but this sounds like a default date and the text implies Manby and therefore Theddlethorpe but it doesn't look or sound right. The photographer/library is given a "Mirrorpix" which may be the Daily Mirror. Sadly the purchased version is only as clear as the first photo I posted . I have learnt not to trust wartime captions as they are subject to tinkering for security reasons.
My thinking is that the date is nearer 1940, although my knowledge of RAF markings is buried deep in my 1970's mind so this may be a clue to the earliest date possible. My other thought is that it was one of those quick ranges set up by an FTS in 1939 of which Fenns Moss (Flintshire), Dee Marshes (Lancashire) and Innes Links (Morayshire), all floodable land ans all had small 12 foot targets originally.
Having now led the witness, does this make a difference to suggestions?
The photo appears in Osprey's Blenheim Squadrons of World War 2. Unfortunately it doesn't give any info on the where and when it was taken but does state it is a two-number code (so 51) belonging to a training unit (perhaps OTU).