This clever little thing is a SunTour Power Shifter, which I picked up on Ebay because it was going cheap. In the land before index shifting, this guy was king. I have the two-lever version on Major Tom and they work really nicely, pulling back with only the resistance of the springs inside the derailleur parallelogram, making a satisfying ratchet sound (Listen here) as the spring moves over the teeth. Push forward and they’re like any other friction shifter, except that it’s the ratchet wheel that’s being held by the screw rather than the lever. They’re less likely to come loose than standard friction shifters, and after a little while you can subconsciously count the ratchet clicks to know where a particular gear is. “Very trick”, as Guy Martin would say. Above all though, it’s just a lovely thing to hold in your hand.