[Arran]

 What power output tubes are used in the power supply/amp chassis? The 37-690s used 6G4Gs I believe, whilst the 38-690s used 6L6Gs. I would not rely on that alone, however, if someone was unable to find the 6G4S they may have decided to update the chassis to use 6L6Gs since they are much more common and used in the 38 model anyhow.
  From what I heard the tuner chassis used fewer tubes in the 38-690, though as everyone is well aware Philco was notorious for producing running changes under their various code versions, as well as during different production runs, I don't know if this was true with the 690s but it was with the 116 models and everything else down the line.
In terms of using power supply/amp chassis meant for a 38-690 in a later production 37-690 this was not typical of the way Philco operated, what would be more likely would be for an early production 38-690 to get the late production 37-690's chassis. If it is out of of 38-690 then perhaps the original power supply failed and someone either bought a replacement chassis from Philco or salvaged a replacement from a junker 38 model.
Regards
Arran