I have a 1964 4" Nickel and the underlug is hollow. I've always been under the impression that the underlug became solid in 1965.The hollow lugs were a feature of the early Pythons. In the past on this forum there was much discussion on when that feature was discontinued.
The likes of your gun will never been seen again. Do you know how to pre-index the cylinder when closing it to avoid a turn line? Hold back the cylinder latch and 'locate' the cylinder so that the bolt only contacts the cylinder notch leads as the cylinder is closed. Rotate the cylinder in a clockwise direction after closing.
A properly timed Colt will never exhibit a cylinder turn ring if it is always pre-indexed upon closing. No other American made DA can claim this.
Regarding how to pre-index the cylinder when closing it to avoide turn line, I want to thank you for talking about it. I thought I was the only one that does that. It makes me a little less like one with an OCD when I learn that other people do it to a Python that deserves such treatment, too