Reminds me - not a tool but the seatpin for my username Peter Mooney. Peter delivered the frame with a Campy pin but using it with a brake cable hanger was miserable. About 3 years in, my 1967 Peugeot UO-8 bit the dust. That plain steel pin and nut has been seen service on the Mooney now 35 years and very well, thank you. (I did cut it down to the proper length.) 12mm nut. It's a regular nut, not an acorn nut, no chrome. The bolt has the perfect nub for the cable hanger. Keeping it straight and adjusting the seat is a joy. I keep expecting it to go away from rust but it doesn't look very different now than 50 years ago.