the 7 is Boings commercial aircraft designation, military were 2, 3 or 4. 5 was pilotles a/c and 6 missiles.
7 on each end, rather than 700, 740 was a marketing idea. no-body knows where the 717 went, again probably marketing, now its the re-vamped MD-80.
not a spotter, Just read 21st century jet, very good book on the 777.