Why is such a high serial number stamped on the lens when only a few were ever made?
We only read of one race tracks use of this super specialty lens in the article. However, I would imagine that there were more than just a few made, and more than just a few are still being used for photo finish work. Take the number of race tracks in existence, and barring a competing lens being used manufactured by Nikon, it's plausible that dozens of this lens were made and exist.
Just because the serial number stamped on the lens is a high number could have very little meaning. It could represent some sort of code much like the Canon Date Code. The serial number could identify the lens to Canon as a certain series, lens type and production specimen as a whole. There's really no way to know.
One has to wonder how many other super specialty lenses exist for specific applications that are not listed in the general Canon/Nikon lens catalogs due to trade secret reasons or due to contractual non-disclosure obligations?
Canon, Nikon and even Tamron have whole industrial optics divisions that produce lenses that are not listed in there general consumer lens line ups. I don't find it impossible that there are other monsters out there with wide apertures to be discovered.