[Arran]

I think inside those nickel plated copper caps were practically Mershons, as inside they are the same plus using liquid electrolite. Likely Sprague first simply copied Mershon technology (or acquired it).

Mike;
  All of those wet electrolytics are designed in a similar manner, only differing in terms of the anode plate design inside the capacitor can. What I was trying to point out is that not all of the 1931 BabyGrand models used copper can wet electrolytics made by Mershon, some used Sprague cans. If a model 90 does not have a pair of copper Mershon filter capacitor cans does not mean that they were not original to the set, Mershon caps were OEM but so were those nickel plated Sprague cans.
 The story I read is that the wet electrolytic capacitor was invented by a Ralph Mershon, so it's very likely that he and his company not only produced wet electrolytic cans but also licensed other manufacturers to produce similar cans, perhaps Sprague had such a license? Then again it may have been along the lines of skirting Edison's patents on a light bulb, like where Reginald Fessenden reworked the filament structure inside a lightbulb so Westinghouse could manufacture them. Maybe all that had to do was change the anode design, materials used, shape, electrolyte formula, or the shape of the can, and Ralph would not have had any say?
Regards
Arran