Yep, that's right. 45 cartoons submitted to The New Yorker or Esquire lost in the mail or in someone's files.
I did check all the New Yorker covers from the 1955-1960 period and I've attached here the only one with lots of canoes. Alas, there are no serenading males that I can see ;) Maybe this is not the cover he's talking about. If you want to give it a try, go here: