Retrieved: 12/06/2013
From marlinowners.com, a photo tutorial on how the bolt, lever, and lifter mechanism works in one of the Marlin leverguns. It doesn't say which one.
Basically, it's a sliding bolt with a falling block that locks the bolt in place. Unlike some of the levergun innards I've seen, this layout looked DIY- able.
The front of the locking block appears to have a slight taper, same as some falling block actions. I don't see why it would necessarily be any weaker than a regular falling block.
In one of the rolling block threads I proposed using a built-up receiver with side plates, that would let you do simple mill work and avoid tedious filing for a square hole. That method, applied here, would make it easier to arrange all the internal parts. (note: pinned barrel like an AK or G3; not appropriate for a normal threaded barrel)