For ARNG O6/COL promotions, how do you interpret a scroll designation of S13-23?

Your assumptions are likely correct. Working backward, the 23 is for the year, the number before the dash is how many "S" scrolls there have been this year through the date of your scroll. The "S" is the type of scroll. Not 100% positive, but "P" usually means MAJ/LTC/CW4/CW5, "L" is for 1LT/CPT/CW2, "S" is for just COL as far as I can tell, "U/D" is for 2LT/WO1 initial commission. Flag officers don't often post on rally point that I've seen, so I'm assuming they have a more direct line to the process. Plus, they are all on hold anyway. Senator Tommy Tuberville (R, AL) is using those promotions as political ammunition for a non-related issue.

I gathered all this from deduction from previous experience and past posts. I do not have any inside contact at NGB or Army G1. I am waiting on P25-23. That should be at SecDef now or soon. It has been about 6 months. Though I've heard a couple CW5s recently that were over a year. Not sure which boat you will likely be in.

If you have any more updates, please post on the current most popular thread here:
https://www.rallypoint.com/answers/where-is-my-fedrec-scroll-in-the-dod-process
Good luck, Sir!