As you can see from the link that Yoyo has posted, the "letter names" are a system used by IPS Publications, and have become something of a standard reference, because IPS publish one of the best guides on the "21 types" VR tests.
It is called the IPS Method & Technique book, and you can buy it here:
Alternatively, using Yoyo's link, you can cross-reference the letter/number types using this equally useful free download from The Tutors:
- Scroll down to "11 plus downloads from The Tutors"
- Click on the link for the PDF "Verbal Reasoning Method & Technique Course".