Actually somewhere in the cobwebs of my mind I think the Pan Am B707 did visit Glasgow on EI flight back in late seventies.
I remember that there was a period of Industrial action that went on for a number of weeks and EI grabbed any avaialable aircraft to continue some operational flights.
I certainly remember the Montana flights whistling in on some of the evening EI flights into Glasgow from Dublin (were they not B720's?).
I am sure EI also utilised other operators during that time (it was winter period I think right enough).
Nivsy