Thinking about this some more, the Colt Police Positive and Police Positive Specials also had positive internal hammer blocks at this time.
Smith & Wesson also had an earlier version of the hammer block (but one that was found could be defeated by dirt, dried grease, oil, etc., which led to the development of today's positive hammer block during WW II)
The earliest Iver Johnson transfer bar ad I've found is from 1904. Lots of variations on the "Hammer the Hammer" theme, but also a lot of ads touting the safety of their double-action only revolvers (often show with children playing with the gun. Imagine the outcry today were those ads to run).
"Must have fallen out of a coat pocket."
That's my guess.