Funny, that doesn't look like an opening phase position. :) It looks to me to be solidly in the DT window.
Regarding the Nactation, I suggest
21$-63Z-62a-62U-43c-63Z-21H-63Z-C?
I'll diagram and comment on the characters I changed:
"H" tends to refer to hitting on the highest point (with down default for unstated half of the play), so here 62H might be interpreted as 24/18* 13/11. For the intended play of 11/5 6/4*, I consider 62A to be much clearer; strictly it means 11/5 8/6 (closest to 6pt), but that's not a logically constructed candidate play. If you have no real aversion to lower case letters, 62a seems slightly better (than 62A) because it is entirely unambiguous.
Another problem with H in this case is that a reader may think you're assuming the obvious deuce (hitting on the 4pt) and that "H" is referring to the less obvious half of the play (and indeed with 21$-63Z-62 the double hit is a good play too). For most double-hit plays, and here, too, I would only use K, but I can imagine someone carelessly using H for it. 62h is better, because it addresses the hit on the lower point and therefore there's only one red herring alternative instead of two.
I don't fault you for using C here, because the 4-cover is so obvious. I'm clarifying only because some related situations are less clear. Strictly speaking, C means 24/21 8/5 (larger split or closer to 6pt), so for 24/21 8/4, I use the bullet-proof "c."
Strictly speaking, R means 18/9, though it's hard to critize because the 6 is so obvious (and it can therefore be assumed that the R refers to just the 3). Even so, I prefer to use a letter that describes the entire play when it's convenient. For that purpose, S, Z or B works well here. (The extension of the back quadrant to a player's entire far side is not yet in the tutorial, but I've posted the concept here a few times.)
Of S, Z or B, I would use Z here only because it appeared previously in the same move string, but any of the three are perfectly fine.
There's nothing whatsoever wrong with your 63D, as it can only refer to 13/7 (the entering 3 being forced). I just couldn't resist changing it to a third 63Z so that each one maps to a 7pt checker.
Hope that helps.
Nack
