American Machine-Made Marbles says that Fisher of Vitro and Howdyshell of Marble King developed veneering at about the same time. (p. 95, top of right column, and p. 153, lower left column)
I don't see a year given in the Vitro section but in the Marble King section it emphasizes that this was in the 50's, as it would have to be if MK was doing it at the same time. The Vitro section mentions Blaine Lemon saying "that it took four years to get the veneering process perfected ...." (p. 158, top of right column)
In the Marble King section (page 95, bottom of left column), Howdyshell is quoted as saying that Marble King started veneering in 1956 or 1957.
So that's more than 10 years after Conquerors were introduced.