The external gears are for screwcutting and the internal gears are the 2 speed high / low drive.
Both machines have the same gears, you just didn't bother to read and compare correctly.
Arc Euro quote
Quieter machine since spindle gears no longer required

MHS was on about the C3 the above quote is for the Supec3 which is the same as the clarke.

I'll have anothe gander at the Taig now?