There are around 11,000 registration certificates that are not online. They correspond to dead trademarks that have no legal standing. I was researching tool companies that held some of these missing trademarks. I requested copies from the patent office through a patent librarian I know. On one request I mixed up the serial number and registration number of the trademark I was interested in. The former number is not that useful and the latter is all important. On the other end of my request was an intern at the patent office. When I met her in person, I related the tale of how she schooled me on my rookie mistake. The group around us burst into laughter, it turned out she had been a teacher at some point and schooling people was nothing new to her!
Mike White’s excellent US Trademark number guide succinctly explains the trademark numbering peculiarities brought about by the Trademark Act of 1946. When my son was younger, he liked the cartoon Ben 10 which has a Null Void- somewhere in dimensional space that you don’t want to wind up. One of the side effects of the 1946 Trademark Act was that it created a Null Void of registration numbers (for reasons too complex to explain in parenthesis, registration numbers 444,812 through 500,000 where never issued). The request I was schooled on was a request for a “registration number” 493,259, which was never issued. The registration certificate is online, where its serial and registration numbers can be seen if you don’t interchange them. (Some of the above is excerpted from this article.)