[Phlogiston]

The metal tubes from 1936 were the first whole year of production. They were constructed differently as noted by the lump near the base/side of the tube and mostly having stamped tube numbers on the steel can. I am always looking for these as most large manufactures adopted them at first only to change back to glass the next year/years. A good 5Z4, in the can with holes in the side, is almost impossible to find. the others were:

6A8
6C5
6F5
6F6
6J7
6K7
5Z4

Besides the 5Z4, these early metal tubes may have been less reliable than the later ones. It is probably better to use later ones in post 1936 radios.

6D5 was proposed