I too was pleased to hear that this definitive book was being written and would appear in a year or so. That was a couple years ago. I have heard that this book has been a year or so away from publication for a decade or so. I think the writers may be serious collectors who are buying all the rare enfields they can find before they publish their book, and so make these rare marked Enfields worth even more bucks.
That said, I think the general belief is that the Sinclair Hamilton stamping consists of a crown symbol / SH / C (centered), or (2) crown symbol / S (centered) / HC. The key is that the lone letter is centered over or under the pair. The mark of Union agent Schuyler Hartley & Graham is a crown / SH / GX with the G being directly under the S and the X being a number ( have seen 2 or 3, and heard 1 also is seen.
Sometimes the Sinclair Hamilton mark has an arrow symbol under the letters.
There is also the even more rare mark of an SH over C in a circle, stamped on metal or (reportedly) wood.
I have an 1862 Tower Enfield 3 bander which as the SH/C mark on wood, and the SHC in a circle on the metal of the barrel, a pretty rare set of markings.
I will
Jeff