Danby, Moore and Herford were three philo-semitic Christian Hebraists who were active from the late 19th century (Moore and Herford) until the mid-20th (Herford died in 1950 and Danby in 1953). All three were responsible in their own way for respectfully introducing and promoting the study of rabbinic Judaism to a larger audience, whether by means of translating rabbinic texts, or writing popular books or article.
This was indeed a departure, for before scholars of their kind the study of Judaism was generally subsumed under the wider heading of Bible, which obviously included Christian studies and basically excluded rabbinic Judaism, ie, anything happening in Judaism in the past 2000 years. This meant, in effect, that Biblical Judaism was studied and interpreted as part of Christian studies, appropriate to be sure, for Christian studies. But it lacked its own identity. As for rabbinic Judaism, its study took place under the umbrella of Semitic or Oriental studies generally, which is sort of a way of labeling it miscellaneous.
I would say that their most famous work in this regard is Moore's three-volume "Judaism in the First Three Centuries of the Christian Era," Herford's "Christianity in the Talmud and Midrash," (but see also his translation of Pirqe Avos) and most especially Danby's complete English translation of the entire Mishnah, which was the very first one. (See the end of this edition, which contains as an appendix a translation of the Vilna Gaon's "Rules of Uncleanliness" from the Eliyah Rabbah. )
In any case, what was a mystery to Wouk, how these guy's mastered rabbinic abbreviations without having a grandfather like he did, really isn't that much of a mystery. While it's true that many a Christian Hebraist had a go-to rabbi or talmid chochom for help, there are actually a number of tools which were available to the scholar who never built his chops in the beis midrash.
For example, the pioneering work in the field (which even has its own name, Abbreviature) is Buxtorf's De abbreviaturis hebraicis (here you can see editions from 1613, 1696 and 1708). This very famous work was twice expanded, once by Tychsen (who has "semicha," see here) in 1768, and then by Gottfried Selig in 1780 (Compendia vocum Hebraico-rabbinicarum). Here's another supplement to Buxtorf. I think Wolff's Bibliotheca Hebraea lists many of them, although they're probably lifted from Buxtorf. These books were all very much available to Moore, Herford and Danby).
There not only are, but also were books by Jews which gave the key to these ראשי תיבות. For example, there is Kethoneth Yoseph : a hand-book of Hebrew abbreviations, with their explanations in Hebrew and English, for the use of students of the Oral Law and rabbinical literature by (1887) by Bene Israel scholar Joseph Ezekiel Rajpurkar:
and here is a sample page from his book:
There is Stern's ראשי תיבות of 1926, and there are many others. None of this is to take away from the accomplishments of Moore, Herford or Danby, but I'm not sure they must have spent years studying with expert Talmudists. In fact, I wouldn't be surprised if Herman Wouk's grandfather saw such a book or two, though not in Latin or English.
See this English Hebraica post.
