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what does it actually DO??]
The piece that projects downward from the lifter/carrier blocks another cartridge from exiting the magazine while the lifter is raised/raising another cartridge up to chamber level so the bolt will feed it into the chamber.
IOW, it is the cartridge stop.
Marlin made a hole in the bottom plate, for it to protrude through when the lifter is in it's lowest postion - w/o the hole, the entire receiver would have to be made deeper to hide it, and therefore also bigger/heavier.
And then, YOU wouldn't buy one, and have your curiosity arosed.

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