I think Matthew is referring to the two serial number ranges for the AI version of the 400/5.6*ED lens.
The lens made its first appearance as a prototype, I think at photokina in 1971 or 72. This was marked as "NIKKOR-P Auto 1:5.6 f=400mm". Note that is was not a "P·C" lens so is not multicoated. It also lacks any "*ED" marking but the literature mentions special glass with fluorite-like properties. In other words, ED glass. At this point Nikon had not yet developed its own ED glass, the preset Nikkor-H 300/3.8 from the same year used ED glass from Schott, so it is likely the same glass was used for the 400mm lens.
This lens made it to market in early 1973 as the "NIKKOR-P·C" Auto 1:5.6 f=400mm". The "P·C" engraving indicates that this lens is multicoated, and by that time Nikon had probably developed its own ED glass for this lens. Nikon hadn't yet indicated the new glass on the lens so it does not have the *ED marking or gold band. At the time, this was a very advanced lens, a powerful yet compact telephoto with special coatings and glass.
In 1975 the cosmetics were upgraded to give it a new modern look. This is the K or "New Nikkor" version. I suspect optically and mechanically the lens is substantially the same as the previous version. The optical diagrams for both versions look identical and both accept the same AI conversion ring.
Production of the K version continued to 1977 and Nikon state the series ended at number 261177. They must have had a lot of parts which hadn't been used for the K version which they used for the AI version because the AI serial number starts directly where the K finishes, at number 261178. This is unusual, most other lenses are given a new serial number in the transition from K to AI. This series continues to a high of at least 261634, a total of 457 units. It's only at this point where the AI lenses are given a new "AI" serial number starting at 270001 and continuing to at least 270398 for another 400 units. Except for the new serial numbers, there is no difference that I can see between the early and late series.
Regarding the post saying 10000 were made, if one ignores the gap between the 26xxxx and 27xxxx series then it could look like there are 10000 in the range, but it really is much less common than that. Or the post was confused with the AI and AIS IF-ED version, of which around 11700 were made in total.