Alex,
My research notes have him making his original visit in 1929 and expanded to 18 holes in 1939. I could never confirm this. The lack of early mentions in any resource is perplexing - as you have said too. It's been so long that I can remember whether its from Bob Moote's list (he made a list of everything Stan did) or James Barclay that was the source of the note. Probably James. It know it was not Cornish, I just checked.
Anyway some notes below about the 1950's work:
"Waterton will definitely have at least the start of a new golf course by next year if the powers-that-be in Ottawa decide to go ahead with Stanley Thompson's plans. 'The expenditure will not be too great,' Mr. Thompson told me. 'Cape Breton National Park's course cost close to a quarter of a million dollars because we had to clear away so much rock, but Waterton will not be nearly so costly inasmuch as I plan to keep five or six of the holes already in operation and inasmuch as there will be few if any difficulties with rock'....."Mr. Thompson was told by Ottawa to come to Waterton and make plans for a new course. 'I am positive,' he said, 'that the only thing that can stop you from getting a lovely golf course now is a war'."
Lethbridge Herald - August 31, 1950
"Stanley Thompson, internationally famous landscape and golf course architect, whose talents have earned him a very profitable livelihood, has been in Waterton for several days making plans for an entirely new layout located on the site of the present golf course."
Lethbridge Herald - September 2, 1950
"Mr. Thompson...spent the past week in Waterton conferring with park officials and putting finishing touches to the alteration plans he began several years ago. 'Golfers who expect the transformation to take place overnight are sadly mistaken,' he said. 'The work will be going on for several years at least before the course looks the way I want it to look'."
Lethbridge Herald - September 13, 1951
"Stanley Thompson, the famous golf architect, was in the park recently supervising preliminary work and laying out the new holes"
Lethbridge Herald - September 14, 1951