Birmingham was the workshop of the world, even in the 80's when I was working in the Auto industry there were hundreds of these multi spindle machines around. My first job was to convert a shop of 20 of these machines from HSS to Tungsten carbide tooling.

I believe the domed head was for both clean lines and less likelihood of a bolt head catching on a branch etc, a lot of off road riding at weekends in the UK. Just a guess.