HO Rolling Stock
This photo very clearly shows the 1959 lineup of HO sets. It is interesting to note that while Gilbert 5 digit numbering began in 1957, the Hudson and 0-6-0 Switchers still bear 3 digit numbers. The numbering on some of the cars in the set lineups may also be 3 digit versions. I have not been able to find any instances where it was used in advertising.