The 242C is a 100 watt general purpose triode similar to the 211D and 242A with a mu of 12.5 designed by Western Electric. The second example was marked ALTEC, but was probably made by Amperex. Altec was using 1500 per year in 1945. The third and fourth examples are marked Machlett ML-242C and General Electric GL-242C, but their construction would indicate mid 1940's United manufacturer.
Length = 7.90" Diameter = 2.60"
Max voltage = 1250
Max current = 150 ma
Fil voltage = 10
Fil current = 3.25 amps