You are not ever going to find a ready-made wrench for that. The
welded-up suggestion is a good one. Another approach might be to use a
die grinder to cut the other 4 flats on your nut & turn it into a hex.
The nut being brass would make it more easily done.
However, fitting a wrench to that nut is likely to be only the first
problem in fixing that shower. After 60 years, that nut is not going to
come out gracefully and there's a good probability that something will
break in the process.
The right way to fix the valve is to replace it. I know, it's in a
tiled wall, but they make large plates to cover the hole. Google/images
"remodeling shower cover plate". Ask This Old House did a segment on
it:
https://www.thisoldhouse.com/how-to/...d-shower-valve
You might be able to fix the old valve, but you _know_ that it won't be
the end of it. Do it right, replace it, and be done with it.