Given the timing of it, I don't think RD stands for Recruitment Depot. I think Regimental Depot is more likely, but since he transferred there after the formation of the RAF on 1 April 1918, 'Regimental' is less likely to have been used since that was very much an Army term. On the other hand the RAF hadn't had time to create its own identity so might well have clung onto understandable terminology for a short period.
The usual abbreviation for Casualty Clearing Station is CCS, not Stn which could easily be confused with a railway station. While the RAF are now located on RAF Stations, I don't think this terminolgy was in use in 1918, for the same reasons as given above. I think 43 Stn is much more likely to be a mistranscription of
43 Sqn.