The AT-257C/4E27 is a 125 watt dissipation pentode usable for full output to 75 MHz. Lewis Electronics was purchased by Aireon in 1946 and for about 6 months produced tubes with the AT prefix. Although the shape is different, the plate cap is different and the base has holes, the 4E27 designation was also used although the standard 4E27 is only rated at 75 watts. Lewis seems to have started making the AT-257/4E27 (75 watt dissipation) without the zirconium anode plating. The envelope seems to be shared with their tetrode the 4DX1. Advertising in 1946 illustrates the AT-257C looking the same as the old Lewis 4E27.
Length = 6.00" Diameter = 2.50"
Max voltage = 4000
Max current = 150 ma
Fil voltage = 5
Fil current = 7.5 amps