Kiltie
As one who has operated extensively using Morse Code, I believe that the use of "R" originates from operations in that mode. "R" as sent by a Morse operator means "I have successfully received and decoded all of your last transmission", i.e with no charaters lost by QRM or QSB (fading). It thereby implies that the message has been received correctly. However if the operator is receiving coded messages he or she has no way of knowing that the message has actually been copied correctly and therefore has to rely upon skill and training to make a best judgement. If there were any doubts then "AA" (All after . . ) or QSL? (Please confirm I have received this correctly) would be used, and in the case of QSL then the received data would be transmitted back - heavy going in Morse! The use of "R" (i.e. Roger) in voice communication is rather meaningless in my view, as only a readback can guarantee 100% accuracy. That is obviously impossible for every message exchange, so leaves the meaning of "Roger" down to trust, like "Wilco"! After all, one can easily understand a message but actually hear the wrong thing!
Regards
OM