One thing that makes modern cycles so desirable and expensive is their componentry. Increasingly electronic, they are more difficult for the home mechanic to fix. Happily, this rear mech. from my early 1970's bike, is definitely just mechanical. It's a Campagnolo Record, the best in its day. Ernesto Campagnolo was a 1920's racing cyclist who later came up with mechanical  improvements and ideas for bike parts.
Extra: My late evening stroll made me come over all abstract.

Many thanks to HOtamer for hosting.