This is a pack of 6 x solid brass rivets and washers in '8 gauge' size.
They measure 19mm long in total, with the stem being 17mm long. The base and washer measure 11.5mm across. The width of the stem is 3.9mm.
They are fitted in exactly the same way as copper rivets, 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.