I am not. It seems this is a thing captured only by part of the English-speaking press.
Besides it doesn't make sense technically they had one chassis of a different construction from all the others.
I agree that "pure monocoque" appears to be an English-speaking press observation. It may be the case that Henry/Lyons/Windsor compared notes about Ferrari cars for their magazine reports, but the "pure monocoque" description is repeated for several races. Detail and terminology (U piece versus rectangular section) varies but the basic story is the same. Perhaps somebody at Ferrari was spinning a yarn?
I'm reading the Sal Incandela book to see what he says about the 312T family and I'll add a summary shortly.
I'd have thought that building the prototype chassis one way with variations for later versions was normal.
Incidentally, Regazzoni, my interest in Ferrari chassis relates to the old story that Ferrari were more interested in engines than chassis. That seems rather simplistic to me, indeed ridiculous given that the 312B flat 12 engine was designed to reduce centre of gravity.