How many different kernel models and implementations do exist?
There are definitely at least two major versions for the kernel PLL: The model for NTP version 3, and the model for NTP version 4. Maybe there is a model for older versions too, but I don’t know. In addition to these major versions there are minor variants of these models. Unfortunately these minor variants can’t easily be distinguished, because their inventor and chief designer, Professor David L. Mills, did not like version control systems or version numbers in the past (see also Q: 6.4.1.4.).
