Kevin Ko gave a really nice description of this strange system:
Your shifter was part of a system which included an aluminum chain carrier that bolted onto the inside of the rear Campagnolo dropout ala the two small holes. The chain carrier acts an auxiliary cog sized and shaped to accept the chain as it is shifted off the smallest freewheel cog. The idea was this allowed faster wheel changes. By keeping the chain indexed to the dropout, the rear wheel could be removed and a new wheel installed with no messing with the chain. After the wheel change, the rider needed to be pushed in order to shift the chain back onto the freewheel. The small rounded lever kept the main shift lever from over shifting onto the chain carrier until a wheel change was needed. The rear triangle needed to be respaced to accommodate the chain carrier.
I have a NOS chain carrier and tiny bolts, but no shifter. +++++++++++++++++++ Now, I don't want to appear cynical, but good buddy Ken Sanford refers to Cinelli Bivalent hubs as a solution in search of a problem; one of their features is that the whole freewheel stays with the frame (spline-fitted to the hub). If that's the case, is the Portocatena to be considered a poor man's Bivalent? :-) Probably cost less, might have worked better, but didn't seem to last as long on the market.
harvey "bivalents are funky" sachs
mcLean va.