Here is one description. Colt called their electroless nickel plating Coltguard. They offered it on the Python, Trooper and Lawman revolver models. After they introduced stainless Pythons about 1983, they apparently discontinued Coltguard on the Python about 1985. It is usually a rather matte finish compared to the bright electrolytic nickel plating. http://www.imagineering-inc.com/solutions/electroless_nickel.htm
I have a 1982 catalog that introduces Royal Colguard on the Python. It is the same as the "standard" Coltguard except for the fact that the Python is more polished than the other models, the same as Royal Blue. Since Python revolvers were always more polished, this makes me wonder whether Coltguard was available on the Python at all before this, but I do not know for sure, not being a Python follower. Hopefully, someone can verify if it was an abailable factory finish in 1972.
Thanks for the information and link. The link says it is superior to other nickel processes and usually more uniform in thickness. I was looking for a 4" Trooper MK III, but may consider this Python since they want $850 for it.