One keeps digging. Manifestly most pilots u/t cannot see through a canvas hood. Aha! What does Sherlock Holmes say?
It is an old maxim of mine that when you have excluded the impossible whatever remains, however improbable, must be the truth.
(The Sign of Four).
Well then. Since these chaps could not see through the canvas, and since I see no peep-hole in the canvas, they must have had an in-cockpit presentation, and here as so often Google does not fail. The TIGER MOTH AIRSPEED INDICATOR MKIXB of 1940 is graded from 0 - 160 “AIR M.P.H.”, and the Australian Pilot’s Notes show at the back a panel in different format that shows “AIRSPEED” (“Circa 1938"). From all this I infer that the early aviators did their simulated IF just as we all did it.
For the sheer joy of VFR, of course, the real pioneers swear by the old Quadrant Machmeter.