Photo © Miguel Snoep
On the photo above you can clearly see PH-BUE has plugs in the upper deck windows and somewhere in the 1980's (?) these plugs were replaced by windows. Did those planes have the upper deck lounge?
Photo © Kjell nilsson
On this photo the left side upper deck door is missing and later on suddenly a door has been built as seen on the photo below.
Photo © William L.B.J. Dekker
Why and where were these extra doors built?
What other structural changes have these planes been through?
And how do they differ from the 747 SUD's except for the SUD and the engines?