Please correct me if I'm mistaken, but I believe you are asking two separate questions: one for Nactation, and the other for the fourth roll play you diagrammed. I'll answer the Nactation question below.
Rich: After 43S, is 65R correct nactation for 20/9? How would one distinguish 24/13? Is that "r"?
Nack: I assume you mean that if Player A plays 43S (and is not hit) and on the next turn that same Player A rolls 65, how do you distinguish the Nactations for 20/9 and 24/13?
Your implied answer is correct. R = 20/9, and r = 24/13.
The position in question is shown below, followed by the position after each of the two 65 plays. (22N was White's play in between Blue's 43S play and his 65 roll, but it could have been almost anything that didn't hit.)
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| 43S-22N-65 | |
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The third diagram (51S-21S-62) and thirteenth diagram (Blue's 55) in this annactated game sequence address the use of R/r in more detail.
Cheers,
Nack