Cleco (pronounced Klee-coe)
A special type of spring-loaded fastener which temporarily holds two
or more pieces of sheet metal together. The cleco pictured here
is of the "hole" type. It can be inserted into a rivet hole and
will hold the parts together until they are permanently riveted.
Clecos come in different sizes, based on the hole size. They also
sell cleco clamps, also called edge clamps, which have spring-loaded
jaws to hold sheets together sort of like a really strong clothespin.