All Through Training Scheme (ATTS)
was a scheme in Britain run by a combination of the RAF, USAAF and civilian schools to combine pilot training from EFTS (or whatever it was called in the UK) through SFTS at one base.

The pilots that went through this system, some 7000 plus, really liked it.

Its proper name was British Flying Training Schools (BFTS) but it was more commonly called ATTS and kits were stamped with ATTS.

I have to admit, his posting to it seems out of sequence. Its possible his role was in some sort of training based on his past experience.
It's not likely he would have been posted to mechanic duties as the ATTS used single engine aircraft only and his background looks over the top for this sort of duty.