A- On the grounds of Fleming Mill
B- On the former Ogilvie farm
C- At Rocky Beach
For the answer ...
Yes, there really was an airport in LaSalle in the 1920s and '30s. In fact, it was there that George "Buzz" Beurling , the famous World War II pilot from Verdun, first learned to fly. It can be located on this 1931 BAnQ map, at the end of the hydro-electric dam wall in LaSalle, which bears the name of Île Gardner. To the west and next to that is Île Seaton, and it is opposite that island that the LaSalle airport was located.
The site of the airport then became a golf course… on which there was an old hangar with a sign marked: Aéroport LaSalle.