[Arran]

Knowing the way Sears worked they, or their manufacturing company Colonial Radio, probably chose the double ended types like the 12A8 and 12K7 because they got a deal on them. Since the set also uses a 12SQ7 rather then a 12Q7, and all of the single ended octal 150 ma tubes were available at or around the same time as the 12SQ7, then this must be why they went with those in the other sockets. I'm surprised that Paul  and 35Z5 didn't notice the the set also uses a 35Z4, that is also an odd tube, I don't think that I have ever worked on a set that used one of those, 35Z3s yes, but not a 35Z4.
 I did run across one of these Silvertones once, at a swap meet, but I  lost interest after I noticed that someone had cut a square hole in the back of the cabinet, so I didn't take note of the tube lineup. It's a shame as it was one of the white ones with coloured bits mixed into the plastic, if not for that hole I would have probably made an offer, even with the hairline cracks it had.
Regards
Arran