The BC-458 is a part of the SCR-274-N command set used onboard the B-17G vintage 1944. It is a 4-tube unit, containing an oscillator, indicator, and two transmitter tubes. High voltage is supplied externally via a BC-456 unit, shown elsewhere on the site. It covers the frequency range of 5.3 to 7 megacycles on the shortwave band, although it can overtune up to approximately 7.2 MC, despite not being marked. This allows for the SCR-274-N to cover all frequencies entirely without gaps.
The set is a single modular transmitter unit which could be placed in a set of two or four. On a typical B-17, the set would have been matched with a BC-459 or BC-457, which cover frequencies on either side of the BC-458. On later variants, a single VHF set (such as the BC-458) would be accompanied by a UHF set (up to 150 MC, now in the VHF band).
As with all of our SCR-274-N equipment, as well as our BC-348 unit, we would like to thank Mike, who runs his own site here: AAF Radio. Without his knowledge, we would not have the functioning and accurate sets that we do.