What does the ".R34" stand for in the Library of Congress call numbers for Terry Pratchett's books?

I've fallen down an autistic rabbit hole with regard to Library of Congress call numbers (mostly because I'm looking for ways to organize my home library).

On the LoC's site, I searched for one of my favorite books: [_Thud!_, by Terry Pratchett](https://lccn.loc.gov/2024008855). It says the call number is "PR6066.R34 T49 2024". I searched a few more of his books and found similar call numbers. Most of the parts make some sort of sense: PR6066 is in the "English literature, 1961-2000" section (while _Thud!_ was published in 2005, the first Discworld book, ~~_Equal Rites_~~ _The Colour of Magic_, was published in ~~1987~~ 1983), and the Cutter number for "Thud" is T48, so T49 might have been to adjust for other books? (Other Discworld books share everything up to that point and generally have the Cutter number for the title there.)

But the ".R34" part is what confuses me. I don't understand what it's supposed to represent. "Pratchett" would be some prefix of ".P718", and I don't see anything else that would abbreviate to ".R34".

What am I missing?

(EDIT: Why did I think _Equal Rites_ was the first Discworld book?)