Since now I have knowledge of such 7-digit 43-86mm zoom lenses with the following serial numbers (23 lenses, 15 of them in my collection): 4399951, 4400001, 4400141, 4400201, 4400371, 4400471, 4400501, 4400541, 4400551, 4400751, 4400841, 4400851, 4400901, 4401171, 4401261, 4401281, 4401341, 4401621, 4401681, 4402171, 4402281, 4402331, 4402851.
Notice: All theese serial numbers are ending by the digit «1»!
My guess: the error happened at the end of the 439xxx serial number range. The correct number of 4399951 would be 439995, 4400371 would be 440037, 4402851 would be 440285 and so on. Conclusion: less than 300 lenses came out of the factory with a 7-digit serial number instead of the normal 6-digit serial number.
Theory: an employee of the Nippon Kogaku factory engraved the serial numbers between 43999x to 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 serial number and the following engraving «Zoom-NIKKOR» is smaller than on the normal six-digit lenses.