[Arran]

I think that the combination of tubes with different bases had a lot to do with whatever the manufacturer could get in quantity, or get cheaply. I have a few Philco sets from 1950, the U.S version was called a 50-524, in Canada they labeled it as a model 84 for some strange reason, and those sets used three different series of AC/DC tubes, loctals, octals, and seven pin miniatures. I think that there may have been a valid engineering reason to use the larger tubes for the rectifier, and power output, if only for heat dissipation, but they could have skipped using loctals in the rest of the chassis, other then the fact Philco probably had a warehouse full of them.
There was one line of sets made by one manufacturer that strangely never switched to seven pin tubes at all, and that was Arvin in their metal sets. You would think that in such a tiny radio that they would have come up with a new circuit, using the seven pin tubes, maybe even made them a proper AA5, but Arvin never did, they used the same four tube setup from 1946 right until they quit making them, in 1956 I think.
Anyhow the Telechron (G.E) radio is a TRF of some description, I guess they figured that these would mostly be of interest to city folk, and nobody would be band scanning for DX. Without finding some engineering memos we may never know.
Regards
Arran