From the description, size and your probable date - black alkathene.

I have exactly the same stuff.

I lopped off the naff transition fittings from various bodges down the
years, and fitted an all-plastic MDPE stopcock (brilliant thing, light
to turn and shuts off to the last drop).

http://www.cityirrigation.co.uk/acat..._Stopcock.html

MDPE fittings will go straight on - no pipe insert required.