Correct. Peter Jewell did the first work on Kilda, but was unable to transform it into a sustained long term project of marked individuals. After he (and John Morton Boyd and Peter Grubb) finished, there was low level somewhat haphazard monitoring of the population through the 70s and early 80s.The first study on Soay sheep was by Peter Jewell, from 1959 to 1967 not 1930s and there have been many more since run by various groups.
I also know 100% that Soay sheep were shot on the islands in the 1960s and 70s.
It was Clutton-Brock who had the insight to realise that the real value lay in a project spanning life times, with marked individuals. More critically, he had the political nous and fund raising skill to make that happen. He got the project as it now exists going in 1985, and it’s run with a consistent protocol ever since.