Bookbinding tool maybe

Bookbinding tool maybe - detail

Ursula Mitra, fellow conservator in private practice here in NYC, and an old friend (remember making phase boxes together at Teachers College in 1990?!?) sent me these images of a possible bookbinding tool.  She acquired it from a bookbinder who didn’t know what it was.

She describes it as follows:

“I discovered a tool, possibly used in bookbinding, but I do not know what it is used for. It is a wooden frame,  8″ x 10″, not glued, so it can be taken apart.  The wood is smooth and not porous (not Oak and not Beech).  There are two pointed blocks that slide freely on each of the short sides of the frame (four total).  The wooden rail that they slide on has a trapezoidal cross section.  Each block has a nail driven into it to facing into the frame it and a string tied to it which connects to the block on the opposite rail (two strings total).  There is no way to lock the blocks and prevent them from moving.  The threads are longer than the frame is wide (10″) and they are approximately 12 gauge or slightly thicker.  The tool may have been taken apart and reassembled in a way not consistent with its use.”

I think she might be right that the blocks make more sense reversed, but  I have no clue what this tool might be.  Any thoughts?