[Arran]

Skip;
  The Mershon caps are wet electrolytics, in fact some of the earliest electrolytic filter caps available on the market, therefore they are polarized, they were 450 VDC ones most likely. However they are large enough that you could  restuff them by installing non polarized poly film caps inside the cans. I think that the tops have removable caps, so this makes it easier to hide your work, the anode and rod will have to be removed from the inside of course, then you just replace them with a brass bolt or threaded rod. 
Regards
Arran