Pete,
Almost certainly drainage, IMHO. If you read "Golf Has Never Failed Me" there is a pic of a punch bowl green, with a caption that it is doomed to failure. Three chapters cover drainage (one called drainage, drainage, drainage - although that might have been editor Ron Whitten's title, not Ross).
In another spot he mentions that you can add contour to a green, but it seems separate from actually locating one. Ross was a really practical guy, and a frugal Scot. No doubt he tried to build courses to last.