CCG,
The lower breech block uses a Lane type extractor, which also serves as a cartridge stop for the ammo feeding from the mag. tube.
That model variant does not use the Stapler cutoff system.
That model cutoff system is mover to the top of the receiver and serves also as the cartridge guide. If you wanted to stop the feeding from the mag. tube, you would move the hinged guide to the right, and it would engage the Lane extractor before the action was completely open. Stopping the feed from the Mag. and allowing the firearm to be single loaded.
To me, it looks like someone tried to make your carbine into the earlier M-1865 Spencer.
Have you gotten a copy of Marcot's book on Spencer's yet? It will help you gain a better understanding of the original Spencer firearms. I can't say how much it will help with the Armi Sport.
My best,
Blair