A year later I re-read this classic. The first time around I missed the most likely explanation for the typeset captions and dialogue. The reproduction (probably from a Xerox copy) is quite poor. I think the folk at "Digital Comic Nostalgic Creations" re-lettered the text that was too broken up to read. For that I thank them.
The story is even screwier than I remember. I don't understand why Havelock & Co undertook their convoluted plan in the first place. They could easily have manipulated the "feeble-minded" heir into letting them loot the estate without turning him into Bigfoot, and without sacrificing a (presumably not feeble-minded) nephew for a test run. It's like swatting flies with an antiaircraft gun. There's a reason Edgar Wallace is not known as a Realist author.
Link to the book:
Super Detective Library 006 - The Door with 7 Locks