Alex,
I don't think this little unit of 4 planes 2-P51s and 2-P38s was ever "on the books" of 515 squadron. I suspect the whole purpose to try and evaluate whether the current planes in the AAF, particularly the P38, were at all suitable for night intruder operations, while they awaited the arrival and then acceptance of the P-61s. The P-61 were arriving at about this time in Europe and had some problems, as is discussed in this link.
http://en.wikipedia.org/wiki/P-61
The AAF in the Pacific was also doing modifications of P-38s to try and get an effective night fighter/intruder.
"If" the T Gates in charge of this experiment is in fact the Thomas Gates later of the 357th it would appear they had a very experienced man in charge. Thomas Gates was trained as an Army Pilot in 1931, worked for the CAA as a flight (and aircraft?) inspector in the 30s then went back into the AAF during the war.