Noting this thread is strictly about the R209 Mk.I, its worth noting that this first version of the set did not have the IF output facility - despite the discussion above of the use of that facility for RTTY. There is a picture of the Mk.I set at:
https://www.radiomuseum.org/r/mil_gb..._set_r209.html
Lacking further info, I presume RTTY was only used with the R209 when the Mk.II version arrived.
Richard
So far I can't see any sign that the Mk.I set was modified for RTTY use,