I am impressed you found quote so quickly. I most likely learned of the concept by reading Bowers myself. But it would have been in a book published after 1999. I must admit that I have looked for water spots since first reading of them, and I have never seen anything I suspected was one.
I looked at the mintages post 1925 as you pointed out. They do indeed, have an "eyeball average" of under 2 million/year. Out of curiosity, I looked at the next largest contemporary (1926-1935) coin, the Walker half. In several years prior to 1934, none were minted at all, and of those issued, the mintages ran mostly just under 2 million, much like the
Peace dollar. In the years 1926. - 1933, the
Red Book does not even list values at the ms65 level, and ms63s are listed at $800 - $3,000. PCGS price guide lists the value range at ms65 at $2500 -$7500, a range much higher than common date Morgans and
Peace dollars. Interestingly, when mintages of Walkers skyrocketed in the 1940s, a correspondingly higher number of high grade coins were minted, and an ms65 can now be found for under $100. Even Ms67s are easily found for under $500.
Of course, this merely proves your point by replicating your analysis. It was meaningful to me, however, as I have a much greater appreciation of how uncommon ms65s are, even in issues of around 2 million coins. Just as important, I will no longer have the Morgan-conditioned feeling that I am somehow "settling" when I buy myself an ms63.