I see from other YouTube videos of Denver that he also played the F-212xl, and other 12 string brands as well. So, chaz might be right that the axe is not the F-612, but rather an F-212xl. Whatever, he was a great one, no doubt, whatever he played. I have my own version of "Country Roads" which I play on my F-412.
The difference in the F612 I believe he is playing and the 212XL and F412 Special he played is that the F612 Is wider, Deeper, and Longer scaled than the others. Not all F612's got the same Checkerboard binding or checkerboard rosette for that matter. While F612's all shared pretty much the same dimensions as each other there are inherent visual differences that I have seen between the 3 different batches of 6 each that were made. Different bindings and different rosettes not to mention different pickguards as well.
Just for giggles Here is the size comparison between the standard jumbo 12 string and the F612 so you can see the difference and compare to the video. You can see in the picture how much bigger the F612 Actually is compare to the standard Guild Jumbo sized F512
One other thing to point to this being a F612 is when the video was taped. This was taped in 1972. John had already been playing a F612 at this point for a few years as his first F612 was one of the first ones made on custom specs from Carlo Greco. His second F612 was from the 1971 batch (same as mine) if I recall correctly. By 1972 he would have almost certainly been playing the F612 for the majority of his performances. Most of the performances I have seen him play with the F412 Special and the F212XL where from the late 60's.
Regardless this is still my Favorite all time performance of this song!
TX