Plumbing solder is 50/50 Tin/Lead.
Electrical solder is 60/40 Tin/Lead.
The very early soldering (with the giant pokerirons that you'd heat in an oven or fire) would probably have been done only with the half-and-half.
I'm of the understanding that the ideal ratio of tin to lead in solder (for electronics work) is actually 63% tin, 37% lead, but I'm unsure why.