Don't forget that in the 70's EI did a lot of lease work - it wasn't just the 747's, but the 737's and 707's were often leased out during this period as well.
I remember that they had a United Airlines 737 flying for them in the 70's for a year or so and I think that this was to fill in a gap of lease that went on for too long of one of their own 737's. Does anyone remember this situation ?