Function follows form...
The tach drive on the Norton Mk III is on the right side of the M/C (as is most that I know of), the speedo drive is on the left side of the rear axle the instruments tend to reflect this arrangement.
On my 1970 Triumph Bonniville the tach drive was on the left side of the engine the speedo drive on the right side of the rear axel, the instruments tend to follow this arrangement.
For a quick reference refer to the Old Brits web site, the Andover Norton exploded views can anawer a lot of questions about the arrangements of the parts and their respective assembly sequence.
RS