I also have an old round body 6P, and the newer Z41 is the same diameter but about 2mm longer than the original. The rubber button is also checkered and protrudes about 5mm out, where the original was smooth and barely protruded.
Thanks for your help on this.My 6p rides in a vertical blade tech so round size is important but length will not matter. Also i really prefer the big protruding rubber button for ease of access.