At one point during this story the clues lead Pons and his dimmer-than-Watson sidekick to The Soames Museum on 16 High Holburn Street. We are told that
The building housed the one-time collection
of the late Sir Rowley Soames, and such pieces
as had been added by various donors since his time
Now, this is obviously the Sir John Soane's Museum, the famous London landmark,* located at Lincoln Inn Fields. The usage Soames/Soane's may be a simple slip, and the change in address perhaps forced upon Derleth by the exigencies of the story (Pons has to crack a cipher to learn the location). But why Rowley instead of John? Did he simply not know the given name and popped something in, not having time in those pre-internet days to look it up? Did he simply not care? Or were the changes deliberate, just as his most transparent minimal changes made the Solar Pons stories publishable in the first place without interference from the Doyle estate.
I'm inclined to pass this off as a piece of local color by a Wisconsin author trying to re-create an ambiance of 1920s London, on par with the frequent descriptions of the English countryside which the person being spoken to cd see for himself. But I can't shake the suspicion that Derleth is putting in a proprietary touch here and there.
*I've enjoyed enjoyed both my visits to the Soane, once to view the collection and once for a wedding reception. A friend of mine (a major Tolkien scholar) worked here for many years.
UPDATE: I've fixed the typo. Thanks David