[Ron Ramirez]

...why did Philco use the enclosed bases in the 1936 models, use the open shield bases w/clips in 1937 then switch back to the enclosed bases in 1938?

Well,

in the 1936 season Philco was still using the standard base tubes. All of the small signal standard base tubes (not counting audio output tubes or rectifiers) have the same size base and easily fit into the tube shield base; size was not an issue with these.

Beginning with the 1937 season they switched to octals, and those 1937 tube shield bases made it easy to use metal tubes as replacements.

Now as to why Philco switched back to the 1936 style base for the 1938 season, we can only guess because this is another of those questions that only dead Philco engineers can answer positively. And dead men tell no tales.

So here are my theories.

A. Philco did not want people using metal tubes in their radios. The animosity between Philco and RCA could have been the basis for such a move (a dumb move when looking back through history).
B. Philco discovered that the bases of the small signal G tubes were the same size as the small signal standard base tubes and, possibly still having an inventory of 1936 style tube shield bases, switched back to save money.
C. Both of the above.

I would pick C. By using the 1936 tube shield bases, they could use up their old inventory of bases and keep anyone from using metal tubes as replacements. Or, at least, make it extremely difficult to do so.

Of course, we know that many of these were bent open, cut open, or even removed so that metal tubes could be used.