This is a 3M Scotch 23 Rubber Splicing Tape, a premium-grade, self-fusing electrical tape.
It is designed for electrical insulation and splicing applications for voltages up to 69 kV.
The tape is made of ethylene propylene rubber (EPR) and can withstand emergency overload temperatures up to 130°C.
It comes in a 3/4 inch by 30 foot roll and is conformable for void-free, moisture-resistant seals. [1, 2, 3, 4]