Hello Tom,
I saw your thread on this subject over at
the other place.
First of all, let's discuss the "C" which does not stand for "Canada" but instead stands for
Compact. Bob has already given you the link in the post above which details the various letters Philco used for its cabinet styles. For example, "B" stood for
Baby Grand which could be a cathedral or tombstone cabinet. Manufacturers did not refer to their cabinets as "cathedral" or "tombstone" in ye olden days. For Philco,
compact was their way to quickly describe a small,
compact table model radio cabinet.
Now as for the "D". I am assuming that this "D" is separate from the 39-6C stamp. Frankly, it could mean anything...or nothing. Maybe it stood for the cabinet inspector (Don? David? Daniel?) or the actual manufacturer of the cabinet. I will look later and see who made the 39-6C cabinets. Some cabinets were made by Philco in-house, but many were farmed out to other cabinet manufacturers.
Really, though, the "D" is irrelevant to the radio. What is important is the 39-6 which identifies the radio, and the C which identifies the cabinet as a
compact table model radio cabinet.
I'm afraid that this "D" may very well be one of those things that, in order to discover its meaning, you would have to ask a dead Philco employee about. And as we all know, dead men tell no tales.
Or, as The Rock would say,
"It doesn't matter what the D stands for!" 
(And he would be correct!)