[Ron Ramirez]

Mike raises a good point. In 1937 Philco models, the tube shield bases were wide enough that any tube could be used, whether ST glass, metal, or the (then-as-yet-undeveloped) GT styles. If the sockets of those tubes have pin 1 grounded, one may use a metal tube.

Now, Philco closed that loophole in their 1938 model radios and went back to using their 1936 model year tube bases, through which only ST type "G" octal bases will fit. This is why we find so many mangled 1938 model year tube bases these days - so repairmen later on could force a metal or GT tube to go into the tube base. And, yes, some tube bases were even ripped out by "repairmen" for this purpose.

Many RF and IF tubes need to be shielded, as well as some 2nd detectors and audio preamp tubes, depending on their physical location on the chassis.

Footnote: Until Philco bought controlling interest in Lansdale Tube Company (and even after that), Sylvania made tubes for Philco, even stamping them with Philco's brand name instead of Sylvania's.