Period 20 is pretty self explanatory, bikes & parts 20 years old or older, yes you can mix & match within the period just as you can with all classes. Revolution is for 20 year old or older bikes with the option to fit non period (later) parts. For example if a 2001 KTM 250SX is presented in standard condition it's a period 20 bike (letter P on the race plates). If the same 2001 KTM 250SX turns up fitted with cone valve forks & Trax shock, then it's a Revolution class bike. If it's presented as Revolution class (letter R on the race plates), all good. If it's presented as Period 20, the rider will be asked to change the letter P to an R & be bumped into Revolution class. Revolution class has evolved as these bikes already exist, there were LOTS of bikes fitted with current YZF/CRF/Ohlins forks/front ends & presented as pre'90/'95 bikes at the 2019 PC Nationals. Those bikes are no longer "cheat" bikes as there is now a class for them to compete in. It also preserves the earlier classes as being period correct with now a class for non period correct bikes in which to compete (no one has to put their bike on the trailer).
There's no reason you can't ride your Period 20 bike in Revolution class if you want more seat time, so benefits all round.
K