I suppose I'm too picky about words, but there really is no transition at that point. One day a particular model had no number stamped in the yoke cut, the next day it did (sort of). No change to the gun at all.
Some have said there was a change in metallurgy at that time, on all or most of the models. So far as I have been able to determine, only the magnum guns (Combat Magnum and .44 Magnum particularly) got some added (or modified) heat treatment, but that wasn't related to the model stamping, just close in time.
So, yes, there was a phase-in of model stamping, with some models getting the numbers sooner than others. Also, within a model line, some guns with lower serial numbers than others got stamped, while others with higher numbers still were not stamped with the model number. But I see that as a phase-in of the new practice. There isn't any transition in parts, which is usually how we collectors use that term. (For example, in 1948 there was a "transition" from the long action to the High Speed hammer. Shortly before that, things like the extractor rod, hammer pivot stud, shape of the thumb piece, etc., changed. But those minor changes don't constitute a transition.)
Anyway, there is my two Yen worth of thoughts.
