First of all, let me say that I personally know zip about the conceptual drawing of the GD, other than it is depicted, as you said, on pages 6 & 7 in "The Book" (at least that is where it is in Hayden's reprint - my original copy is long disintegrated and gone!). I at first thought you were talking about the overall schematic drawing of the final layout on pages 84-85 and couldn't imagine what all the fuss was about. But, this morning my curiosity got the best of me, so I turned to page 6 and took a closer look at it and, so - voila,!!! I now understand your interest.
I think it would be useful to list what we know about the drawing (like I said, I know nothing, other than it IS in the book). A number of questions (and one observation) come to mind:
1) Is it an original by John, or did Linn draw it (or have it drawn by a Kalmbach artist to demonstrate the overall operational scheme)?
2) When was it drawn?
3) Do we have evidence that it still exists? Why would it?
4) If it IS a JA original why did he draw it? It's actually a pretty detailed representation of his imaginary system so I don't think he drew it on a napkin while his operators played with the "Timesaver! Do any of the remaining operators have a recollection of seeing it displayed anywhere in his house? Was it just a large doodle he did to show visitors what his layout was all about?
5) Did he have any other layout diagrams displayed in the house or down in the layout room?
6) Have you approached Bob Hayden about any info on the drawing that he might have come across when he was republishing the original?
I'm sure there are other pertinent questions that it would be nice to know the answers to, but my second morning cup of coffee is empty, signaling it's time to be about (in the spirit of Sherlock Holmes). But, first - my observation:
I had never really looked that closely at the drawing before, but I did this morning. And, while perusing I spied something that may lend some info to the speculation surrounding "The Great Missing Devil's Gulch Bridge Mystery". Look closely at the depiction of the bridge by the artist (whoever he was). If this WAS John Allen's work I would say that at least at the time of this drawing's rendering he was thinking in terms of an almost mirror image of the Scalp Mountain Arch span (adjusted for length, of course) from SM to Angels Camp. To me, that makes so much more sense than the suspension bridge suggestion, that has always seemed a bit goofy to me - wouldn't such a structure completely overwhelm the entirety of the mountain scenery? On the other hand, maybe it WASN'T John who did the rendering and that whoever did just made the bridge design up! I doubt we'll ever know.
Well, enough Saturday meandering - off to "honey-dos"!