We were somewhere around Barstow, on the edge of the desert, when the
drugs began to take hold. I remember Gordon Henderson
saying something like:

Googling finds various names for plastic pipe - mostly seem to refer to
blue alcathene or MDPE pipe, but I'm not sure that's what I have. It's
black!

So can some kind soul tell me the name/official size of the pipe and
suggest a coupling device?

It will probably be black alkathene - is it thick-walled, ie, 1/2" bore
in a 3/4" outer diameter, or metric equiv? The thin-walled stuff is
commonly used by farmers for shed and field cold water supply.
If thick-walled, you can simply treat it like copper pipe and use a
normal compression fitting on it, tightened well. I have a 150psi mains
supply with a compression fitting on it that's survived a decade like
that.