I intend to replace an old bottom bracket, but having no expertise in bicycle mechanics I can't quite figure out what type it is. The bracket has a 127.5mm spindle, is possibly made by Shimano, installed in the early nineties. Both cups have a regular splined inner edge; the left side cup is metal, with a lip that is flush with the outer surface of the BB shell, and bearings can be seen within by removing a rubber seal ring. There are threads at the inner visible end of the spindle on the left side, terminating in a cylinder with a rounded slightly hexagonal shape too deep to access with a spanner. The right side cup is plastic, and has no notable features. I suppose that's the "adjustable" end.
Neither end of the BB really looks like the few pictures I've seen of cup-and-cone brackets, but the bareable bearings don't fit what I know of cartridge brackets. Furthermore, from what I read the adjustable cup is usually on the left side, not the right as it is here. I've got a replacement cartridge BB waiting in which the sides appear to be reversed and the bearings completely inaccessible.
Is this a cup-and-cone or cartridge BB? Did someone install it the wrong way around or are bottom brackets with the adjustable cup on the right commonplace? I tried preliminarily opening both sides after a WD-40 soak, spline tool fastened with a long bolt and turning in the direction of pedalling on both sides, but the rings wouldn't budge.