patrick conroy said:

So when I lop off the 15A 115V plug - I suppose I'll see three wires. Black,
n' white, n' green.
When I attach the NEMA 6-20P plug, he said redundantly, does the black need
to go to one particular blade, or can I choose thanks to the magic of
Alternating Current.

It doesn't matter. As long as the green goes to the ground pin.

I ask 'cuz thanks to the magic of the internet, I learned a few years ago
that there was a right way to wire zip cord in a lamp with a polarized
plug...

It does matter. ;-)

FWIW,

Greg G.