UPDATE:
Hours of searching has lead me to realize that the base gun IS in fact a Winchester Model 1866 'Yellowboy' 24" barrel.
Not entirely sure if the original has the octagonal barrels or not, but this version by Uberti's does, and so does Doc's. So that's a match.
Secondly, I know it is this model because the trigger and bolt pins on the outside are in the same spot:
Plus, it's the only octagonal barreled Winchester that has the same pins (that I have found).
Doc DID in fact extend the barrel:
See where the tube magazine weld point is? There's an extra 1.5 feet or so of barrel added. The base rifle is 43inches, and Christopher Lloyd is 6'1".
You can see here by the front sight where there is the SAME weld point. Looks like they just took another rifle barrel and connected them:
As far as the big loop lever, I found one for $64:
I'm sure you could find a better deal.
Still no luck on the telescope. So far, I'm still thinking it was a scratch build prop.
Any corrections?