This is an unusual and rare bicycle tool made by a machinist in Southern California and sold in the early eighties to bicycle shops who used it to convert Campagnolo Nuovo Record and Super Record double cranksets into triples (the smallest chainring these accept is 41-tooth; not very small).
The jig is held in place by the bottom bracket axle and you can then accurately drill and tap 5 holes in the spider. Into these holes you insert Avocet/Ofmega triple-conversion adapter bolts. These are steel and are mounted to the spider with locktite if the crank will remain a triple. The adapters have female threading on the backside to accept standard chainring bolts. To install the granny ring, you simply rest it atop the Avocet/Ofmega adapters, screw in the chainring bolts and easy pedaling is yours!
I recently found a new home for this tool with a bicycle club that has several Campy-equipped classic bicycles in need of triple conversions.