Another item I noticed is that the #2 barrels have the reinforced forend lug. This feature is often associated with Long Range and Wildfowl models, but I understand it was an available option on any grade after it was introduced. Th Not sure when that was, but I expect it was well into the 1920's.
As to the extractor function, I like the joint check guns, too. They open farther, as David Williamson noted. I didn't see anyone comment on cocking function for the joint check barrels on hammerless guns. Don't they have the "figure 8" cutout in the forend iron, too? The joint check doesn't have a cocking function, does it? The joint check forend photo posted by Tom Archer appears to be from a Hammer gun. At any rate, the later style barrels/extractors can evidently be fitted as long as the corresponding forend is used with the spur to operate the extractor pushrod.