What a boon this internet is for research and spreading data. Depending on the search criteria you use and how accurate your description is, you can find almost everything you desire.
My collection of almost 900 (17th to 21st) century bookplates was purchased from a retiring antique dealer and friend in Sydney a few years ago. Bruce spent a good portion of his dealing career in Trinidad where he bought the three generations of MacKenzie's bookplate collection. The engraved plate above was one loose item in the spares box. He happened to just show the collection to a woman from Tobago, Trinidad who was herself a Hamel-Smith who identified the plate as belonging to one of her ancestors. Her name was Ann Giuliany. She in fact returned home and later on supplied Bruce with annotated photocopies of her relatives portraits, below - thus expanding the research.
"Several of these bookplates were among letters addressed to Dr T. Hamel-Smith".
"This gentleman might be (?) the mysterious I. Sydney Smith Jnr. His photo was amongst others of the family and is the only one un-named".
"Dr Lionel Hamel-Smith with wife Mabel May Gisle and daughter Nellie (my Mother) about 1893. Dr Hamel-Smith died in Port-of-Spain in 1894. Photo by Sellies Studio P.O.S.
The two cabinet photographs are of I. Sydney Smiths relatives . . .
My further search has not thrown up any additional family crest info on the double cross symbol, but I continue to search it out, any response appreciated.