Yesterday I received this e-mail.
Contrary to what you wrote on your post about the Marshall CC Swiss, you didn't miss a win in the game against Kelleher after 52.b6, because this position is drawn with accurate play from White.
After 52...Ke6 53.Kc6 Rc8+, we arrive at the critical position.
I agree that 54.Kb7? Kd7 55.a4 Rc5! is winning for Black.
That's why White must absolutely play 54.Kb5! with the threat of 55.b7 winning a tempo on the rook. After normal moves like 54...Kd6 (or 54...Kd7, or 54...Rh8), there follows 55.b7 and 56.Kb6.
Such are the joy and pain of rook endings.
On a personal note, I take great pleasure in reading your chess blog. Keep up the good work!
The e-mailer is correct. During the post mortem analysis, my opponent and I were under the mistaken impression that 54.Kb5 would lose to 54...Kd7 55.b7 Rc1. What we overlooked is that White draws by underpromoting to a knight with 55.b8=N+!.