"open" multiple employer plan

When filing the form 5500, what should the IRS plan # be listed as for the PE? What if the PE had a prior plan that merged into the MEP - what should the IRS Plan # be then? What happens when the PE decides to start their own "stand alone plan" - what should the IRS plan # be listed as then? What happens if the PE decides to start their own "stand alone" plan and previously sponsored a "stand alone" plan that merged into the MEP that the PE adopted?

When setting up a "stand alone" plan in which a PE was previously participated, what should the IRS plan number be listed as? They were an adopter of an "open" MEP in which they filed a separate 5500.