@sittingbull - interesting thoughts!
I think you're right about points 1 and 2, judging by the comments I've seen on the web. Relating to point 3, the frames I've seen seem to be mostly 80s, with possibly a few 70s ones thrown into the mix. Judging by its components, which I believe to be original to the first build (all contemporary with one another), mine is c. 1987. So, in terms of active production (or active badging up at least!) Ian May probably existed for 10 to 15 years max.
On to frame serial numbers... mine has F 01370 stamped on the BB. Now I'm guessing that the last two digits aren't a reference to 1970... I wouldn't know how to date from this code.
My idea that there are probably 100 or less left is just a guess. I think in all my research on't web I've probably come across 15 Ian Mays. Since the power of Google is strong, I reckon a ratio of 1 (known) to 5 or 6 (unknown) is probably about right... so, roughly 100.
They aren't all large frame sizes BTW. Mine's a little too small for me (but I'll forgive it that) at c. 21 inches.