The machine (HP i3) - a replacement for an old (now unsupported) XP machine - was bought second hand with a factory reset. The installation of MS Office was clean, without any customisation. So the .DOTM file would/should have been wherever that installation put it, standardly Users>[NAME]>AppData>Roaming>Microsoft>Templ. - where it now in fact is.
Word was stable prior to finding the .DOTM file, so no reason to think it was corrupted.
Yesterday's modifications to the Normal.dotm template seem likewise stable.

PS. You say in your linked web site that you shouldn't put page numbers into the Normal template. I have done so, and find no problems. Can you explain the advice?
Cheers.