Some good looking bikes here...hopefully this thread will end up being somewhat comprehensive of the model.
The SN on the rear DO of mine is: 2993974, stamped very clearly. So, it's at least in the same universe as Unworthy's. Eschlwc's # off the BB shell is something completely different...maybe you'll have a number that makes more sense on the dropout?
Compared to Unworthy's though...his should have a higher number than mine if mine is an earlier bike by a year or so, instead it's the reverse. So, assuming there is a system to the numbers (!), maybe it's not strictly sequential, or there is something else going on. I am struck that mine ends in '974' and it appears likely to be a bike from that year. But Unworthy's ends in '595' and the bike is certainly not from that year! Any chance it ends in '75', Unworthy? If so we would be getting somewhere. Not that it matters to anyone but a few scattered nerds...or just one. One lonely bike nerd.
I didn't bid on that GR on eBay. Too many projects underway. However I did really like the way the paint looked in the photos, and I think I'd even use the term 'patina' for it. My own GR is going to get a full teardown, cleanup, polish, etc, but I hope it will still look its age. A bit of crackling to surface of the paint would be okay with me!
Also, took a closer look at the rear rack: it's a Jim Blackburn, neatly affixed with P-clamps. From that and the Suntour barcons and water bottle holder, I am forming an idea that the original owner of the bike, presumably the one who did these changes, knew what he was about and made the bike into what he needed for serious use.
^btw: a C&V friend here who has a GR says he traded his Grand Jubilee for it, as he could not get comfortable with the Jubilee's extra-long top tube. The bikes were the same height though of course if they weren't the same year specs might have changed, but he thinks there's a distinct difference between the models.