Found this today on a Petromax 828 B. A threaded bushing with threads on both sides.This meant that clay burners of the 200 HK size or the 300 HK size could be mounted This is the first time I've seen something like this and I'm wondering whether Petromax offered this in the 1930s or whether a hobbyist was at work here.
Very interesting, and very clever. I have seen similar on older electric light fittings, but never a lantern.
Reducer rings and adaptors were available for wick lamp burners and for the undermounts which were fitted to the bases of the oil lamp tanks, so your reducer could be a continuance of that idea and therefor a genuine Petromax product, perhaps some paperwork will turn up which shows it.
