I'm absolutely certain I read somewhere that the driver's vision slits on the M1917 were two inches higher than on the FT, because the American troops tended to be taller than the average Frenchman. Have I dreamt this? If not, can anyone remind me where I read it?
Chin chin.
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I may yet prove to be writing in haste, but having had a quick look at photos of the two on Landships II - looking out for the tell-tale differences, as there still seems to be mixing up of the photos in the two articles - it looks to me as if the height varies from FT to FT; some seem to have the lower height slit shown in Charlie's pic above, whilst others seem to have it about halfway up, just like the M1917.
However, I doubt that anyone would dispute the difference in average height.
Side vision slots for the driver were added.
This book may reference the mod, although it's primarily a photo book.
French Light Tank Renault FT & U.S. Six-Ton Tank M1917 - Model Centrum Progres (publisher)
Refererencing Charlie's photos, it looks like the only constant between the two panels, the rivet on the right, roughly halfway vertically, could be used to confirm the height change.
A lot of tech info here, but no mention of the raised front vision slits: