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

ANGLE GRINDER.

Only for a clot.
The wall strength of a cut ring spanner is ****.

Then it was a **** spanner to start with.

I'd agree with you: I've a ring spanner for 1/2" tail nuts that was made by
cutting out a 16mm piece (sorry - not with an AG), sawing off the cut ring
and welding it back on 90 deg. for getting under sinks etc.

It's had a fair bit of force on it at times and is still in perfect
condition.

And what was the wall thickness and depth of this ring spanner to start
with? I suspect it was a fairly hefty old piece.
NP's assertion that it would be 'a **** spanner to begin with' is full
of holes - then again, he talks much ****. Many of the high-quality
slimline ring spanners wouldn't stand for this kind of treatment, but
you'd tend to find the older low-tech, low-browed,
material-instead-of-quality designs might, probably not for long though.
Here's a clue - I have several Snap-On open brakeline spanners going
back decades and without exception they are all hex (not 12-pointed) and
heavy sidewalled to compensate for the open end.

Aye, **** spanners indeed.