I got a Warren and Comice Pear tree from Raintree. The Warren stated Warren Semi Dwarf. The Comice stated something along the lines of Comice SERR though.
You should ask Raintree. It may be an abbreviation or simply an error.
The only use of 'Serr" i know of in the plant world is a reference to Dr Gene Serr, a University of California-Davis professor of many years who was a legend in walnut circles. Professor Serr was internationally known for his work with walnuts before he passed away in 1968. He also worked with chestnut trees, and traveled all over the world to look at fruit and nut trees. A walnut tree and a chestnut tree are named for him. I don’t think he ever worked with pears but I could be wrong.
Walnut was the first hting that came to my mind from the title, but I hadn’t looked to see if I was remembering the connection correctly, or rather the spelling of the connection. (I received random walnuts in the mail once that I narrowed down to a probability of a SERR variety before I trashed them. This was when strange seeds were showing up from China labeled as jewelry or other things.)