No. 57 Squadron Bristol Blenheim code identification help??

[Mark Copeland]

Hello!!
I am researching a particular Bristol Blenheim Mk I that flew in 57 Squadron. It shot down on October 16, 1939 near Lingen. The Pilot was Flying Officer Michael James Casey RAF POW No. 24, the Observer was Sgt. Alfred G. Fripp RAF POW 5752 and Air Gunner J. Nelson. All three crewmembers bailed out safely and were taken prisoner. The serial number of the aircraft was L 1141.
In researching the aircraft, I found that 57 Squadron put Squadron markings (DX) on their aircraft just before the aircraft was lost. The question that I had was to determine the actual individual Squadron aircraft assignment letter. In other words, DX - ?. I want to identify the aircraft letter assignment.
Just too also check facts, the camouflage was the typical Dark Green and Dark Brown with a segmented underside identification marking of black and white divided undersides. The code markings were in light gray and the serial number was carried on the side of the fuselage as well as the rudder.
Can anyone give me a clue as to what the proper codes were for L 1141? I would appreciate any help that you could lend.
By the way, the pilot Fligh Officer Casey was one of "the 50" that were executed by the Gestapo following the "Great Escape."
Sincerely,

Mark S. Copeland