AP, all bomber Command trials were given a number. Usually they were conducted by the Bomber Command Development Unit - BCDU - which had a small unit with, I think, parts at each main base. It may have had one or two aircraft where equipment to be trialled might be fitted but for larger trials, such as 505, they needed to use the main force aircraft.
448 as clearly a flight refuelling trial. The times when the trial took place might be significant or the details of the receivers. Unless a Ppruner comes up with the detail you could ask the AHB giving as much detail as possible.