This dates back to when mountain bikes were first big in the 1980's and remains a good way of getting the bars up high.
Given its mountain bike roots the stem is built tough - forged and heat treated for durability.
Please note - this 80mm extension version has a step machined at the top of the shaft, whereas the 100mm version is smooth all the way up.
Material: Forged Aluminium
Handlebar clamp diameter: 25.4 mm or 26.0 mm
Extension: 80 mm
Post diameter: 22.2 mm