I suspect that changeover period was somewhat 'blurred' anyway, as there would have been several production lines going at any one time, & I believe there is probably considerable overlap. I'm sure there are others that have studied the serials in detail & can better comment (my interest is mainly only in so far as serials relate to No4 T conversions), but I suspect we're looking at a 'transitional phase' of somewhere around 11C to 15C.
ATB.