Hi Jay -
The electrical solder I usually use is 0.031" (0.8mm). I've also seen (and, in the past, used) thicker electrical solder - about 0.062" dia. I've never seen plumbing solder in a dispenser pack like you describe, but that doesn't mean it doesn't exist.
Is it solid, or flux cored? A quick glance at McMaster-Carr shows solid 50/50 solder in 0.062 and 0.125" diameters. Cored doesn't rule out it being non-electronic - they do make acid core solder, but non cored (solid wire) pretty much rules it out as being an electronic solder (again, in my experience).
Could you post a photo of the solder and package in question?
Do you have a temperature controlled iron? The 50/50 melts at a temp about 45*F hotter than the 60/40 does (420*F vs 375 for 60/40 and 360 for 63/37). If you have a temp controlled iron, try setting it for 390-400*F. If the mystery solder doesn't melt, it's likely 50/50.
If you don't have access to a temp controlled iron, melt a blob of 60/40 and a similarly sized blob of the mystery stuff on something you don't care about scorching (a piece of wood or PC board scrap, for instance). Heat each in turn to molten (try to heat for about the same amount of time, and note that if the mystery solder takes longer to melt it's likely 50/50), then stir with a piece of wire or the like as they cool and solidify. The 60/40 should go from liquid to solid relatively quickly, while 50/50 would still seem mushy and moveable for a longer period.
That's about all I can suggest off the top of my head.
-Pat