FL370 had a pretty good explanation in post #740. To embellish....I think the 5 designation arose from when we had LOT bidding, paper bid sheets, paper AE inputs, virtually no computer interface, and most importantly a clear distinction between international and domestic flying...their was no cross pollination between the two and the only way to "cross streams" was bid a new "category" even though on the same plane. The L10 was domestic and the L15 meant international flying for the L-1011. As, mentioned, the 76-400 was the 765 if flown internationally. I think the outdated nomenclature came from entering the computer age for bidding and they had a defined input parameter on number of characters. The computer guru who was elevated to head of IT with his career path being baggage crusher...fueler...gate agent...res agent...customer service agent...red coat ....station manager.....dot matrix printer purchaser oligarch....then promotion to "management" as data entry key punch card inputter and then his elevation to head of IT...all about 30+ years ago. they never wanted or knew how to devise a good strategy for the here and now because they kept putting Band-Aids on the feeble 30 year old system.

Suffice it to say..."it ain't really no big deal" once you figure it out. It is so convoluted that when AE's come out I had to look at the bottom of the AE(where they list the equipment)to figure out the correct input entry.