The Mayfair action is a sort of copy of a George Gibbs action with a side safety on it. It also has been simplified in doing away with the retracting striker and the shape of the original lever catch. I think the retracting striker is quite an important feature as it means if anything goes wrong with the primer the striker will never get stuck in it jamming the action.
The original Gibbs actions are beautifully made but the PD actions are more robust in my opinion. They were made in calibers up to 600 and were used by several well known makers of the time so really are a well proven design.
I'm not sure who makes the brass but can find out. The cases I've used have been pretty good. The brass is a good thickness and they're consistent on the dimensions.