This is a pack of 10 x copper rivets and washers in '10 gauge' size.
They are 19mm long in total, with the stem being 17mm long. The base and washer measure 9.5mm across.
The diameter of the stem is 3.4mm, so it's best to punch a 3.5mm hole in your leather.
Copper rivets are fitted by pushing the stem through a hole in your leather, adding the washer and tapping it down flush with your leather.
Using end cutters you then cut off any excess stem length, leaving yourself with approx 3-4mm to hammer down into a dome shape using a copper rivet fixing tool.
The correct fitting tool for these rivets can be found below in related items.