This is a pack of 8 x solid brass 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 width of the stem is 3.4mm.
These 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.
Solid brass is slightly harder than pure copper.