UA for years ran ORD-LAS as flight 711. CO had 1776 as IAH-PHL but no longer. CO 1492 is back as IAH-CMH. Back in the 1960's and 1970's, Eastern had nightcoach flights that ran southbound and northbound that used 400 series flight numbers, bad example EA 401 JFK-MIA (1972 accident)
For years westbound and southbound flight numbers by all airlines were odd numbers and eastbound and northbound were even. This started changing in the 1990's not sure why.
Flights like 1,3,5 or similar odd single digets were JFK-LAX on AA,UA, and TW.