Many thanks for the note and the link, Peter. I believe I had encountered precisely this discrepancy between old British threads and their modern metric counterparts a few years back.
I suspect Peter will well remember two stunning, restored, red Holdsworth framesets from the late 1950s. When I began to fit each of those, bikes with mudguards, I had instantly met with noticeable resistance. I had assumed this was due to the recent re-enamel, so I did immediately (and cautiously) introduce a 5mm tap. This worked fine to "clean" the threads... which, from recent CR List members' comments, I now suspect was in fact actually enlarging the openings - albeit ever so slightly. In that case, all was then perfectly prepared to accept the modern standard stainless steel metric screws; since then I had given this no further thought.
It was unfortunate that my friend had not gone a bit more slowly with the build up of his frameset. In fact, even more unfortunate that he had not informed me - before the fact! - of his intention to mount newly purchased mudguards. Had I been there, I would have promptly dashed back to retrieve my taps and could have saved him a full day of undue stress, depression self-condemnation. Nevertheless, ultimately with a successful outcome.
And this turns out to be yet another good example to all of us why we should always proceed slowly and patiently - especially with older bicycles. Every single one may offer a host of unexpected... uh... challenges? - and even to experienced and devoted veteran bicycle mechanics with years of experience. And, of course, I am not referring here to myself, but rather a to few local gents who, for decades, have made a career of turning a bicycle wrench. I sometimes present them with questions from my own current projects which often one or more of them are quite surprised to have simply never previously encountered.
It is always nice to have experienced locals to consult, and when this is not an option, this forum is always an extremely valuable resource. Thanks again to Peter and to all of those List members who have devoted decades of their lives to cycling and who now offer us their invaluable experience and knowledge. Your contributions are always received as much appreciated gifts to relative newcomers, like myself, who now enthusiastically embrace this common interest.
Cheers!
Bob Hanson, Albuquerque, New Mexico, USA