On Sunday, March 12, 2017 at 6:41:52 PM UTC-7, wrote:
Back in the 60's and 70s, test leads were coated with a nice soft
rubber, which was thick, had excellent flex...

A 1972 catalog says Columbia
#01319 (red or black) 20 gage, 41 strands #36 AWG wire
#01321 (red or black) 20 gage, 41 strands #36 AWG wire
#01326 (red or black) 18 gage, 65 strands #36 AWG wire

and Dearborn
#982041 (red or black), 20 gage, 41 strands of #36 AWG wire, and
#981865 (red or black), 18 gage, 65 strands of #36 AWG wire

and Belden
#8898 (red or black) 18 gage, 65 strands of #36 wire, 10 kV insulation
#8899 (red or black or green or yellow) 18 gage, 65 strands of #36 wire, 5 kV insulation
#8890 (red or black) 24 gage, 45 strands of #40 wire, 2 kV insulation

all fit that description.

Belden still makes #8899 (Alliedelec.com has stock); a bit over a dollar a foot, 100foot spools