Yokozuna brake pad insert/slip-in for use in the fancy threaded aluminum holders and newer type Shimano brake pad holders. Famous Scott/Mathauser salmon brake pad compound that works great in wet and dry and lasts a long time.
If you're using Tektro R559 sidepull brake, the cartridge aluminum brake pad holders are already included with them so all you need to do to have new pads is slip these guys in.
These are about 54 mm long. Also available in a longer 73 mm length. Measure to make sure.