However, that doesn’t always apply to lenses of different kinds from adjacent batches. On very rare occasions (19 in all, or 0.03% of such batches), the last copy produced in a batch with serial number (n), is followed by the first copy of the lens in the next batch also with serial number (n), instead of (n+1). I have long puzzled over this as an error on my part because it hasn’t been previously reported. I then realized that no one else is likely to have access to this information. I have checked and re-checked myself to be convinced that this phenomenon may be real. It is definitely human error, either mine, or another's (or both) and infinitesimally insignificant. Just in case you think I'm being curmudgeonly, on a good day I believe the error is a keyboard stroke misfire, but the analysis below allows otherwise. The best part of this dithering is that it underlines the truth of the above tenet (and my keyboard needs a good clean).
So, only adjacent batches of different lenses very rarely have the same serial number because of this kind of lapse in continuity. To date I have recorded over 68,000 lenses in over 5,800 batches. In the first 1 million serial numbers I found two pairs of batches sharing a serial number. Then in the mid 1960’s just after 2.5 million serial numbers 6 close-packed examples appear. Only one example occurs up to 4 million; another two around 5 million. Three appear up to 6 million (now including 6x7 lenses); one up to 7 million and four up to 8 million, all involving 6x7 lenses.
I am sharing this information for historical reasons. These observations may follow historical trends in the complexity of manufacture when staff and procedures would be facing greatest demands. But, I have not been able to hold, or observe two lenses with identical serial numbers simultaneously. Actual details can be supplied if needed.
(These comments don't apply to AOCo publications. "Artistic licence", or other rules apply, because serial numbers on illustrations in brochures are often fictitious (i.e. re-used serial numbers).
2021 Frequencies now are posted for M42 lenses.
Regards,
Droo