I think your timeline is a little messed up.
As has been discussed on this board, the "Trouble with Tribbles" diagram actually is from "Space Seed." It was originally made for the script direction of Scene 44 where Khan is reviewing diagrams for a Constitution Class Starship." The close-up of the graphic ended up on the cutting room floor, but the drawing was dusted off and repurposed later for "Tribbles." So the notion that Khan is looking at Constitution Class stuff was scripted long before "Doomsday Machine." (I guess whether we are to assume that Khan was interested in the class of ship that the Enterprise is, or if he's more interested in classes of ships that are forerunners of the Enterprise is something Matt Jefferies would know.)
Bjo Trimble probably landed on this Constitution Class data point for her Concordance book from 1968 from film clips or the script or both.