[Arran]

I have noticed that RCA (actually G.E) developed a whole range of metal tubes, including power tubes, which they heavily promoted for a few years, then either put them on the back burner or discontinued them outright in favor of glass types. I can recall reading in the OTB that they were even contemplating how to make a metal magic eye tube, well metal-glass obviously, but it never got beyond the prototype stage, sort of similar in construction to one of those awful M-G style picture tubes that were popular in the early 1950s . From what I can recall the metal power tubes were not liked because the heat dissipation was not that wonderful, but it may have been that with rectifiers in particular many preferred glass because you could see what was going on inside, like if a filter cap is shorted you can spot that almost instantly inside something like a type #80. I have noticed that metal power output tubes seem to be fairly common, but that was probably because they marketed as being mil spec as Russ suggested, whenever you run into a metal 6V6 or 6L6 they often have a JAN number on them as well.
  I don't think that I have seen a glass version of a either a 5Z4 or a 5T4, and have not seen either used in a radio built after 1938, a 5Z4 is pretty much interchangeable with a 5Y3G or GT, only it has a cathode tied to one of the heater pins, so they were pretty much redundant after a short time. I too have seen the shield used over a type #83, and I assumed that suppressing RF hash was the purpose behind it, but I did run into a series of pre war Grimes-Phonola/Electrohome built sets that had a tubes shield over the power output tube, I have no idea why as it was only a 6V6G or 6F6G under it, the sets were not high end, and the chassis were large, and inside spacious console cabinets. Here is an example:

http://pacifictv.ca/schematics/phonolaz40a62xe1.pdf

 An odder feature is that it seems to employ both a standard tuning condenser but also uses permeable (slug) tuning, and there is no equivalent to Doug Houston when it comes to Grimes-Phonola/Electrohome radios to ask about this that I am aware of. I think it's for band spread tuning on shortwave, but why bother when it doesn't even have an extra tuned RF amplifier stage?
Regards
Arran