At first glance, the serial numbering sequence seems rational; the Model 62 phasing into the Model 99, then on to the Model 116. But note that the Model 62 and Model 99 populate both the 1^st and 2^nd generations. If the change from red to green aperture index occurred at a specific point in time, then both these models were in production concurrently. Otherwise, during the production run of the Model 62 the aperture index color was changed, but then at the beginning of Model 99 production the color reverted back to red before it was changed to green. Neither hypothesis makes much sense. I have no answer.

I do not have, and have never seen, either generation of the Model 62 in black finish. So the next question is: were black finish Model 62s ever produced? Only Tokyo Optical could answer this question and they seem disinclined to make such information available.

The Model 99 spanned the first two generations and was made in both chrome and black finish. The Model 116 includes both chrome and black finishes in the 2^nd and 3^rd generations but the 4^th generation was produced only in matte black, common to most other lenses of this generation.

The Model 116 was of a new design, with internal components that differ from those of the earlier lenses. One major change was the configuration of the aperture ring detent. The older lenses incorporate the same axial design used by other RE Auto Topcors: a flat spring under the dust cover for the aperture ring coupling pin, spring-loading  the little detent ball. The pressure on this ball is parallel to the optical axis and engages a detent plate mounted on the rear barrel. The new design uses a small coil spring to force the detent ball out, in a radial direction, against detent slots machined in inner face of the aperture ring. Some of the later RE Auto topcors, of different focal length, adapted this new design.

According the Topcon Story optical improvements were also made during the evolution of these lenses. But even with disassembly it is impossible to identify, with confidence, the exact configuration of the optical module within the lens. It is for this reason that I choose to use external attributes is designing my classification technique. But this lens especially is hard to pin down and the best any collector can do is check the external features and hope that what's inside has not been corrupted.

For anyone who just wants a lens to take pictures with, all this is unimportant. In general, the later the vintage of the lens, the better the optics will be; but some early lenses are capable of making clear, sharp, images. Another reason to opt for the later 116s is price--the models 62 and 99 were made in only limited quantity and therefore may be more collectable (read more expensive) if the seller is savvy. The 4^th generation matte black lens is also somewhat rare and may be offered at a premiums price.