Glad you shared this - all the other details jibe with my understanding of the Waterford coding, but this is the first time I've heard that "M" indicated the OS tubing. I think I recall one table referring to that digit as a "company code." That never made any sense to me at all until now, so good information!

My '89 is two months younger than yours and a bit smaller. One interesting thing about my 60cm OS frame is that it is has a Columbus SLX decal. So it's wrong on two counts: (1) it's not Columbus tubing at all, and (2) SLX - at least to my understanding - would not have been used on a 60cm frame. Nevertheless, this odd decal is the only peccadillo. It's a truly remarkable bike, as I'm confident in saying yours is also likely to be.