I'm sure the original reason for tinning the cores was to protect the copper from chemicals in rubber insulation. If I recall correctly early PVC insulated cable in UK was still plated, then it occured to someone that this was a needless cost, and bare copper in PVC started.
Exactly. The manufacturing process of rubber involves vulcanisation, which is done using sulfur. The sulfur would corrode copper, therefore any rubber isolated copper wire is tinned.
Many people mistake tinned copper for aluminium.
Even today's rubber cords have tinned copper wires (H05RR-F and H07RN-F are the European designations).
