@JuliaZLSAT PT52 is September 2007, June 2007 is the free PT that LSAT provides so I think they just skipped it in the numbering (or it's listed in some places as 51.5).
I've decided to purchase the .pdfs off Cambridge for 46-51, meaning I won't have to wait for a book to arrive, plus I like writing them like this as opposed to the bulky booklets Amazon has.