These original instruments are an absolute nightmare. The number you're looking for is on the dial, below the word CHRONOMETRIC. The usual format starts with S, then the speedo model number? along with is a number which is the key to the whole ensemble and relates the number of turns of "something" per mile." * Plenty of images on the web. Forum members with accurate speedos will hopefully respond with what's out there. M A represented "Motor Accessories", hence SMITHS M A.
Something per mile as urban myths range from the number of turns of the cable, the number of turns of the back wheel or now more closely the number of £££'s for a good example. Those in the know, know, and this is why a genuine unit costs a lot.
The drive is from the gearbox, so any change in gearbox speedo drive pinion (unlikely) or final drive ratio (possible) affects the reading. But with another unit to hand a comparison is simple, and will show if the problem lies within your present speedo. It is definitely a job for an expert, warnings already on this thread include the fragility of the original dial printing, and you need guile and skill to get the bezel off without damage, then inside there is more precision chaos. And yes, there are EXPERTS and experts. So don't even consider a backyard fix. Indian versions look the part, but the one you have is probably more accurate......
Later models (A65) took the drive from the rear wheel and use magnetic type instruments. A whole lot simpler. Japanese bikes used shorter cable, saved costs, speedo drive from front wheel, genius!!
Swarfy.
*I'm no authority, just what I've picked up over time. Hope this helps.