Warren Lewis made this spool in the 1960s. The drum is a 3 mm thick steel tube. The attachment arms and rope guides are made from 1/4x1 inch steel bar stock. The attachment arms are made of a single piece of bar stock, while each rope guide is a separate piece.
The attachment arms are not the same length. The seat maillon is clipped to the longer arm. If desired, a safety can be clipped to the other, but I haven't found this to be necessary. There is some possibility that the rope will slip off the drum, but this just leads to a jam that can be sorted out, provided on remembered to bring their ascenders (or knots).
This spool is big and heavy, but when one remembers that it dates from an era of 3/4 or 1-inch manila rope, one realizes that device weight was not a big concern.
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©2007, Gary D. Storrick