@TomTheuns @academicchatter @histodons You might find this interesting.
Note the phrase used here to describe the League of Nations: "A guardian of treaties".
https://books.google.com.au/books?id=ad1mAAAAMAAJ&pg=PA67&lpg=PA67&dq=%22guardian+of+treaties%22+%22league+of+nations%22&source=bl&ots=k0VZourEdE&sig=ACfU3U1jIQA81RpiPCWNJbC3GHWgP7tAbw&hl=en&sa=X&ved=2ahUKEwjm5rTLhoKBAxUGslYBHUvtAC0Q6AF6BAgqEAI#v=onepage&q=%22guardian%20of%20treaties%22%20%22league%20of%20nations%22&f=false
It might be fruitful looking further into whether/ how the phrase was used to describe other intergovernmental bodies and treaty organisations?