Original Tristars had DG/VG's no INS.
Triple INS became an option and in later years the early Littons became expensive so there was a mod ( That Air Atlanta did ) to go back to VG/DG but added GPS.
I think the first area nav was s/n 1024 which had the Ambac/Decca system.
Also this aircraft ( and 1032) was originally certified at an MTOW 199,900 kgs ( instead of 430,000 lbs to keep it below the 200K limit for airport and nav charges
Some -500's were digital and normal operators cannot interchange them ( But perhaps 411 has found away). Of course if you go down into the E & E bay they have the same shape size and colour black boxes but us professionals have to go by part numbers which are not interchangable. ( some of them only have one digit difference
)
The FMS units ( by Ham Standard ) were program for individual operators and to re-prrogram cost an Arm and both legs !
There was an Oil consumption limitations on 1 aircraft. This was the King of Jordans aircraft. It was due to very short legs at low altitiudes. The problem was that the oil seals never got fully presurised
Another problem was the fact that it was a very computerised aircraft ( even toilt flush
) and it was very common to find that re-racking the computer solved the problem. However in the early 70's re-racking a computer was looked upon as a fudge so computers were replaced just to make it look right for the Authorities. Most of these came back from the shop with " no fault found "!