Jumper wire in 1980's synths
Looks like 24 or 22-AWG, 7-strand copper wire, "tinned" or solder-coated. 

"Jumper" wires can be any type of wire - insulated, non-insulated, stranded, solid, etc. Most often, solid wire is used, and it is tin-plated to reduce corrosion. 

In Metric units 23AWG would be around 0.5mm. Simply obtain some of this, commonly refered to as "hookup wire", strip off any insulation, give it a slight twist, and tin it with a soldering iron to have exactly what is shown here.