I absolutely agree on variation between examples of lenses, especially as you say when wear and tear is introduced. Lens rentals blog has published work on that which shows clearly sample variation. What I question is wether that variation is more than, excepting abused glass , the difference between an "old C non T* lens and a newer CF T* for example. The contrast and flare difference would I would think (an opinion backed up by no facts whatsoever) be greater than batch variation between CFs.
On charging, please see my earlier post with an example of that practice, for all I know he may be the only one doing that though.
For the OP:
More difficult when distance buying but the lenses can be dated as follows:
You can tell the age of C and C T* lenses from the three or four digit code marked in red on the lens rear barrel. It can be found by focusing the lens to its closest focusing setting and, from the rear, looking inside the barrel.
The last two numbers indicate the month and the remaining one or two numbers indicate the year when 1957 has been added to it.
Therefore a lens with the code 2104 would have been manufactured in April 1978.
The newer CF lenses use a different code system which is found in the same location as the C lenses and is usually in red.
It consists of a one letter and two number code. The letter represents the month with "A" being January "B" February etc. And the following two numbers are the last two digits of the year in reverse.
Therefore a lens with the code H98 would have been manufactured in August 1989.
You may find lenses with more than one code. I understand if returned to the factory for major work, element replacement for example, the work date code would be added.
The lens serial numbers bear no relation to the manufacture date.