I would think any unused 50 year old jig is indeed rare. Mr. Burke's design was later copied by the makers of the "Scrounger" jig which wobbling action was phenomenal, similar to lipped cranks. The Scrounger head is available now at an outrageous price. For those not familiar, the Scrounger mechanism goes beyond a flat or slightly cupped crank lip being a waay bigger deep scoop concave. A brilliant design really, but even back when the cost was prohibitive considering standard Do-it myself designs worked fine.