First of all it looks like it was written in the old careful library script. All 4 lines are textbook library cataloging and classification and it seems that the cataloger even made a mistake.
The number 21101 is the accession number for that particular library. It was the 21,101st book added to the collection. Many libraries don't do that anymore, but it was common for a library to always add the accession number to the same page number in every book they acquired (always on page 13 for instance).
As to why there are no distinguishing marks as to which library owned this book--it may be Holmes' copy and he may have been a closet librarian or another thought is that all of the ownership marks were on the missing slipcase. Philips Academy in Andover MA is the home of the Oliver Wendell Holmes Library, where he once went to school. I don't know if they have a collection of his but it may be a place to start looking, as well as Harvard Law's digital library.