A big feature of the Double Wedding Band model (see here, here and here for background information) is that it incorporates one of the more obvious patterns of the Strike series: the Leapfrog effect, or the tendency of odd numbered books to connect with odd numbers and and evens with evens. The Hallmarked Man continues this pattern
What I am most excited about is those connections that have, thus so far, turned up in all four the even-numbered books. I'm going to call this the "Truly Appreciated" list, both because I was happy to see them and because I think they should top the list of predictions for book 10.
