Amazon listing says the book is set 1914; book itself suggests 1910.
Sign of the Four is set -- according to Wikipedia -- in 1888, at least 22 years before this book. Why in heavens name would Dr. Watson, Sr., think the dog Toby from the earlier book was still alive 22 years later? Had he learned nothing of basic common knowledge in all his years?
No mention of technological advances: motor cars, telephones, electric lights. Pah!
UPDATE: Pg 176 -- mention of telephone for first time.
It's not even an interesting mystery, and recalls too much of Sign of the Four.
I'll finish it -- skimming, I presume -- for Bingo, but this is yet another dud.
Fortunately, the book I've chosen for my last remaining square is NOT a dud. ;-)
UPDATE: Pg 133 reference to "recent" election of H. H. Asquith as PM, so setting is 1910. That the publisher Macmillan could get it wrong on the Amazon listing is telling.
UPDATE: Pg 134 reference to use of the loo. OED apparently gives first published use of the term to James Joyce in 1922, though of course spoken vernacular may have been earlier.