Does MATS mean anything to you? I'm thinking Military Air Transport Service, but I'm not sure that's actually what those letters are.
The other thing is - that wasn't started until 1948. This image is supposedly from a wartime photo.
By the way, I did mange to make it through all 853 pages of that document. There are indeed many squadrons that used a triangle in some fashion, but none I was looking for. For anyone not very computer savvy like myself, I imagine you can do a control F to search for particular aircraft, particular missions, etc... Very useful.