speedo drive angle

You haven't stated year or type. Most drives that turn up are ex-WD16H (Long Bush, left-hand) and for a left hand brake plate. Inters with 21" front wheels probably need alternative gears too.

Can the gearboxes be distinguished by looking at them?  The outer cases are 'handed' two ways.  But are the gears inside also 'handed'?  It looks from the catalogue excerpt that there are 4 options!  Can the correct one can be found by looking at the outer casing only? 

Chronometrics.co.uk should be able to assist.

I believe there are two lengths for the part that enters the backplate.  That must be adjusted (using plasticine or blutack or similar) to make sure the gears inside are properly engaged.

The 21" wheel us a further complications.  An error of 21/19 is about 10%.  Enough to end up in trouble in Cameralands if you forget it.

I don't believe that it's possible to tell without turning them. With drive held as it would be fitted to the machine, and turned as per wheel rotation, the output shaft should always be turning clockwise (except for some late 1930s Enfields that had a reversed drive in the speedo head..let's not go there ! )