Well, I'm too late to help you out here, obviously, but maybe the info will be helpful for the next time you see one.
That's actually the standard location for the left-side roll mark on an older 6" Python -- which is actually my only aesthetic quibble with the older Pythons. I have 6" Pythons from 1957, 1959, and 1961, and they're all roll-marked in that spot, give or take a millimeter or so. If you saw a Python from the 1950s through the mid-1960s with a centered roll mark, that would be reason to strongly suspect that a later replacement barrel had been fitted. (As would the absence of a hollow underlug, although I've read that the transition to the solid underlug in the mid-1960s was sporadic.) As your photos show, though, the roll mark isn't always necessarily in exactly the same place within a given time period -- it can vary by a few millimeters. Why Colt couldn't be more consistent with the location of the mark at any given time is beyond me. The same applies to the Rampant Colt logo, which seems to wander all over the sideplate on Pythons and other Colt revolvers.
I also have an early E-prefix 6" Python from 1970, and the left-side roll mark is perfectly centered on that one. However, it is my understanding that some other Pythons from this same time period have roll marks in the original location. I've read that Pythons from the mid-1970s almost always have the left-side roll mark centered; I don't know at what point that ceased to be the case.
Finally, I have a stainless 1990 6" Python with a roll mark that starts a little before the halfway point of the middle vent and ends just a bit past the start of the vent nearest the frame. I don't know how this compares with other Pythons from the same time period, but I wouldn't be surprised to hear that the placement wasn't consistent then either.
