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.
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