General Cleco pins
Cleco (originally Avdel) pins
... are used in sheetmetal fabrication, for clamping sheets together, through holes, usually subsequently opened out accurately for rivetting. They can be installed and removed very fast... wing nut and hex-nut versions clamp slightly better but are much slower to use. Traditionally, they were used in the aircraft industry to clamp and keep-aligned sheetwork being drilled-off and subsequently rivetted.
However, they can be useful when fabbing/repairing steel sheetwork by welding as well as rivetting, to keep sheets aligned, the job in-place, and the clamping holes subsequently spot-MIG'd up
They come in a range of sizes, colour coded, usually imperial, the 1/8" copper coloured being typically most popular... and can be easily obtained from aircraft parts and tools suppliers pretty cheaply (a lot of a/c tools and parts can be cheaper than many equiv auto tools, as there is less of a 'racket' and more professionalism in that industry... the pliers are about £3-00 and the pins about £1-50 to £2 each.
Other cleco-style products