I think the 5inch speedo (as such) was originally fitment to Jaguars, the Vincent variant (of the Jag guts) had a specially cast aluminium body.
An original Vin fitment had a right angle gearbox at the cable entry, the gearbox was retained by a screw into the speedo body (this is usually a giveaway for determination of 'OEM' versions.
The Auto Tempo versions did not use a gearbox and the drive cable connected straight to the speedo and did not look so 'for the job' as the originals.
I seem to vaguely remember something about the AT version only having one light (the originals had two), but that may just be old age playing me up!