I grew up on paper hulls, they didn't have to actually get "Wet" to swell. They could just absorb moisture from the air over time to swell without actually having been in water
There is an L shaped part pinned to the frame on my Halifax. The vertical leg extends up behind the barrel extractor with the horizontal leg extending back into the frame beneath the rising bolt which lock into the rib extension. I had assumed that when the lever is lifted the dropping bolt hit the top of this lever forcing the extractor rearward. Maybe it doesn't make contact, but I see no other reason for its existence. I
I will have to "Smoke" them up & see if it does indeed make contact to force the extractor out.
With the barrels off I can push that fired hull into either chamber & the spring does not push it back out at all, yet with the breech closed on it, it never failed to push it out that approx 1/8 inch so I believe it is being forced by other than just the spring. Only once with several tries though was the shell pulled out more than that 1/8 inch, so if the spring was doing the work it should have come on out to the limit of the extractor's travel, then on with the breech block from there, but it didn't.