OK, yes now I've seen the script it is clearly Stn. However I still don't believe that is a reference to a CCS. The events recorded are postings or attachments. A soldier (other than one in the RAMC) would not be posted or attached to a CCS, he would be admitted. Casualties were either on the strength of their parent unit, or posted the X list.
Usually a man would only remain in a CCS for a couple of days, and was either returned to his unit if relatively fit for service, or be moved back up the casualty chain to a field or general hospital where he could undergo surgery etc if that was appropriate. The sort of casualties who tended to remain longer in a CCS were those who were sick (STDs were a common ailment and in 1918 so was flu) and were already receiving the appropriate treatment (ie drugs, bed rest etc) for their condition, or all too frequently, were about to die and for whom nothing more could be done other than making them as comfortable as possible. CCSs frequently had large military cemeteries adjacent to them.
I don't know enough about the organisation, structure and terminology of the early RAF to offer a sensible alternative explanation for 43 Stn.
On the question RD, he went there immediately after being with 65 Sqn, which at this point in the war was an
operational flying squadron based in France. So it would not be logical for him to go a Recruiting Depot, but it is entirely plausible that he staged via a Regimental Depot (which is a kind of holding location) while waiting to attend a course at 25 TS (probably a Training Squadron). However I am happy to be corrected about Regimental Depot because it really doesn't fit that well within the newly formed RAF which of course had no regiments. Again I can't offer a better explanation for RD for the same reason given above.
There are two excellent books which might throw some light on all of this, if you can find a copy of one or both:
Rawlings, John D. R. (1969).
Fighter Squadrons of the R.A,F. and their Aircraft. London: Macdonald & Co. (Publishers) Ltd.
Sturtivant, Ray; Hamlin, John; Halley, James J. (1997).
Royal Air Force Training and Support Units. Tunbridge Wells, UK: Air-Britain (Historians) Ltd. ISBN 0-85130-252-1.