PH made a 5E which was for mounting on Mauser actions, the 5E/4 had a different mounting system to allow it to be used on a No4 action.
As far as I know it was only used on the Enforcer as the mounting bracket / position meant that you could not use a standard 'safety'. If using the PH5E/4 then you MUST use an Enforcer (shortened) safety.
How do I know : I built an Enforcer clone and couldnt activate the safety - it was locked in the 'off' position.