Royal Crowns
One of the tools we use to date buttons is the use of Royal cyphers and/or crowns. From these articles it appears that the change over to use of the “St Edward’s crown” started in 1953, but that there was a period of transition.
Note that the St Edward’s Crown is used for only coronations, but there are other crowns. Below is the Imperial State Crown, which is modified for each Monarch to use at other times than the coronation.