It really ought to be easy, since the '02's engineering changes would always be one step away from those of the '05, yes?
The origin of the 'engineering change' designation is found in some documents prepared by Walter Roper, when he was running the service department. He needed a way to insure that guns being returned for service, would be fitted with the correct parts, when necessary. The engineering changes were the source of his problem.
So, he prepared a list, by serial number, that designated which parts were to be used for revolvers in those particular serial number brackets. Because the round butts (1902) and square butts (1905) were in the same serial number series, and were identical guns (except for butt configuration), his list was very simple and straight forward.
I am not sure that he put names on the serial number brackets. Whether he did or or not, there was no need for a name for each bracket.
The collectors picked up on the list, and turned it into a definition of engineering changes by serial number. It was this effort, on the part of the collectors, that causes the difficulty. Namely, its "not nice" to refer to the same gun with two different engineering changes, because the guns are not different (except for the butt configuration).
The way out of this difficulty, as chosen by the collectors, was to do away with one, or the other, of the two models - for the purpose of assigning engineering changes. And that is exactly what happened - the 1902 was dropped from all the engineering change designations, when the cylinder stop was redesigned and the 5th frame screw added. This, intentionally, coincided with the collectors introduction of the model of 1905, at serial number 62450 of the .38 series (it was actually introduced at serial number 58000 as a new model).
The answer to the question is that, yes, the difference is just one engineering change. I've always felt that this is not a palatable resolution - not for me, and probably not for anyone else. That doesn't change the problem, or make it go away. It just means that there is no answer - at least, not yet !
Regards, Mike Priwer