Seeing as how that is for a 6V electrical system and the 6HA6 would also confirm that, is is possible that the missing tube is a 'vibrator'?
If you can find a Rider's or Sams guide or GM stuff on it etc. you'll know what it is and the rest of the tube lineup would help. There are old style capacitors in coated tubes underneath as well (and they can and do LEAK so 'modern' electrolytics usually replace them, even get hidden inside to preserve the appearance) and a look under the chassis would surely help. Old radios are even known to have parts combined in a single molded package that has to be checked and custom repairs made. You don't get so lucky as to just plug a new tube into a 70+ year old radio and it just works, without some servicing.
That having been said, it's pretty cool and will be fun to use when it's finished.