[Arran]

 I would bet that the conversion was post war, those Pyramid brand electrolytic caps were around in the 1950s and 60s. From studying the photos it looks like they kept the socket in place from the 1H5G, since that's also the one where they left the grid cap in place and you can see a pair of hollow rivets holding it in, as for the 1N5G and the 1A7G, it looks like the sockets were either replaced, or more likely, they drilled out the rivets holding the tube shield bases in place so they could use metal or GT style tubes in those holes. Strange that they would have kept the grid cap in place from the 1H5G and eliminated the ones for the 1N5G, and 1A7G, the easy subs for those would have been a 12K7 and a 12A7, but it looks like they maybe have  originally routed the grid cap leads through the hollow rivet holes for the original tubes, though for the 1N5G the lead probably came out of the round IF can on top of the chassis. 
  During the war years it was extremely difficult to find 150 ma series string tubes, the black metal types debut around 1940, just as the war was getting started, and the 50L6 came out in late 1941 I believe. It looks to me like someone did the conversion, probably in a rural area that was electrified after the war, and used whatever tubes they had on hand to convert the set with the least amount of work. Now the question is, do you want to convert it back into a battery set or do you want to restore it as a home brew AC/DC radio? The conversion does not look to bad to me, though I wonder what they used to hold the 35Z3 tube socket in place, I don't see any hardware?
Regards
Arran