Mark S,
Looking at the picture of the rear of your Silvertone, the 42 tube is the audio output tube, the 6D6 would be an IF tube, the 6A7 would be the oscillator/mixer tube. I couldn't find a reference to the 8BX tube. If this tube has four pins, then it probably is a substitute for an 80 rectifier tube. The tube you identified with a question mark is probably a 75 detector/AVC/first audio tube. You may check the radio dial and see how many bands and frequencies it has and compare the tube layout with some of the schematics on this site under Resources. Perhaps someone on here has a data book which crosses the models of radios to manufacturer and tube layout.
Radiodoc