The 638 is identical to the more common 6D111; the only difference is the 638 was made before WW-II and the 6D111 is simply the post war version. There is no significant difference other than model number.

Tube complement: 12SK7 RF, 12SA7 mixer/oscillator, 12SK7 1F, 12SQ7 detector, 35L6Gt audio output, 35Z5GT rectifier