The production of the 43-86mm zoom-lens started with serial number 438611 (lens in my collection). Somewhere at the end of the 439xxx serial number range an additional digit «1» was added on some lenses for an unknown reason.
Since now I have knowledge of such 7-digit 43-86mm zoom lenses with the following serial numbers (26 lenses, 16 of them in my collection): 4399951, 4400001, 4400141, 4400181, 4400201, 4400371, 4400391(EP), 4400471, 4400501, 4400541, 4400551, 4400751, 4400841, 4400851, 4400901, 4401171, 4401261, 4401281, 4401341, 4401621, 4401681, 4402171, 4402281, 4402331, 4402771, 4402851.
Notice: All theese 7-digit serial numbers are ending by the digit «1»!
My hypothesis: An employee of the Nippon Kogaku factory engraved the serial numbers between 43999x and 44028x another time and produced duplicate serial numbers. When they became aware of this, they just added the figure «1» at the end of the duplicate serial numbers (instead of a colon following the serial number they used in earlier cases, e.g. lens NIKKOR-S Auto 1:2.8 f=3.5cm No.920157:). This would also explain, why the space between the last digit of the seven digit serial number and the following engraving «Zoom-NIKKOR» is smaller than on the normal six-digit serial numbers.
The error happened from the end of the 439xxx serial number range to the beginning of the 440xxx serial number range. Lens number 4399951 is a duplicate of lens number 439995, lens number 4400371 is a duplicate of lens number 440037, lens number 4402851 is a duplicate of lens number 440285 and so on. Approx. 300 lenses left the factory with a 7-digit serial number instead of the normal 6-digit serial number.