The quarterly paper that is sent out (I think it is quarterly) contains info on inserts that have been confirmed. That is, it has been verified by a number of reliable sources. There are a number of us who brick search who report directly to him, and from our reports, included with his findings, the lists are compiled. Since he gets the reports from people, there is a general idea of scarcity of notes as well, take HOH 2004 for example. (Not even an insert). Anyway, oddities will show up now and then, and when it comes to inserts, sometimes they do not show up for a while (HOG 9.9).
Anyway, what I am saying is that his letters are very good to get the first scoop on what is what, based on the most current findings.
I find it very helpful.
H