There are elements of fact in it. Those numbers you have quoted are the last 3 digits of the part number which will be stamped into the cluster gear inside the box. 496 is the standard M20, 497 is the standard M21, but there are other M20 and M21 clusters too with different ratios and hence different cluster gears.

Twin rings on teh input shaft on a V8 box used to mean M21, but since the 80's tehre have been aftermarket ones available with 1 or no rings on M21 so it is no longer any help. Plus XU-1 M20 boxes have 2 x rings on original boxes, AND 6cyl M21 boxes are the same (of which there are 2 x different ratios)