I had meant to open a thread on the critically important topic of maker's stamps/marks, and now that some of the recent comments are starting to address this issue, I think it's important for us to keep those comments together for future reference.
Calvert did indeed stamp his Rooks (both of them) rather than the King, to the best of my knowledge. I have two stamped Calvert sets, both of which appear to be fully authentic (by provenance and by close inspection):
http://www.crumiller.com/chess/chess_pages/calvert/calvert1790.htm
http://www.crumiller.com/chess/chess_pages/calvert/CalvertStampedIvorySet.htm
I also have a Calvert-marked box that came with a set, although I am not at all sure that the set was original to the box. I was told that the Calvert marked boxes came with wooden sets, not ivory sets. Anyway, here is the marked box:
http://www.crumiller.com/chess/chess_pages/calvert/calvertivory.htm
Of course we will also need to discuss the sensitive topic of "counterfeit stamps". I have one Fisher set (marketed by Fisher, manufactured by Lund, according to Frank C.), and this set was sold to me as a stamped Fisher set, but in retrospect I am 99% sure that the stamp is counterfeit. Here is the set:
http://www.crumiller.com/chess/chess_pages/lund/FisherStampedSet.htm
...although I do want to say that it is a lovely set.
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