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ColinTheBald
@davidual
Portacatena lever.
Around 1979 Campagnolo designed a system to speed up wheel changes. Pressing that dingus let rear derailleur shift past the smallest cog and onto a crescent shaped chain rest bolted to the inside of the dropout.
Many frames can be dated by looking for the two mounting holes on the dropout.
Portacatena never gained much use and disappeared by the early 1980s.