Here's a sample of GR's handwriting I found:
This memo is reproduced on p. 148 of Inside Star Trek
, and the caption refers to "Roddenberry's handwritten annotations." I actually found it before, but I saw the memo was from Justman so I assumed it was his handwriting. But then I remembered it was in IST, so I double-checked, and voila.
I'm no handwriting analyst, but to my eye they look like they're definitely the same hand. The cursive f
are are similar in both, and the printed "an" in the standalone "man" in the IDIC sketch
looks like that in "planet" in the middle line of the memo's margin notes. Plus they're both in a cramped and messy hand, while the Theiss handwriting samples reproduced in TAoST are larger and cleaner. So my entirely non-expert opinion is that the rough IDIC sketch, despite being credited to Theiss in TAoST, is actually by Roddenberry.
So that would mean Roddenberry came up with the basic triangle-and-circle idea, and Theiss refined it under his supervision.