It couldn't have been a prewar design, because many of the tubes didn't exist then. RCA was able to design the 621-TS DURING the war because they were manufacturing military electronics, and were creating the very latest miniature tube designs, such as the 6J6, 6AG5, & 6AU6. These types were released in 1942 and were originally used in radar & other HF military applications, but were also made available to the TV design engineers. This made possible the 621 design, which had low noise RF & IF stages, 21-25 MHz IF (vs. 8-12 MHz in prewar designs) and the first tuner that could operate on high band channels.