Thanks for the advice @Tenderloin , @rodabod , @freddo & @panic . Really appreciated. My dad has his heart set on a 1958 Seamaster so the hunt is on. I'll keep you updated with what we find.
It may be exhausting, but I'd start looking through the movement serial numbers of every single watch which interests you, rather than searching for "1958". Perhaps you've been doing this already. That's 16M-ish as you know.
Thanks for the advice @Tenderloin , @rodabod , @freddo & @panic . Really appreciated. My dad has his heart set on a 1958 Seamaster so the hunt is on. I'll keep you updated with what we find.