Here's a bespoke aluminium gear knob, ready to fit on your K11 (and I'm pretty sure K10 - but no idea about K12) gear lever shaft, it is counter bored to fit snugly over the shaft where it increases in diameter below the thread and of course threaded to suit. It is 70mm high, and very slightly under 37mm diameter. It feels lovely to handle. This is a one of a kind item machined from solid bar stock especially for you!