A.Y. Jackson’s “unknown” landscape

I visited McMaster University's Museum of Fine Arts in February 2020, to view several early (pre World War I) works by A.Y. Jackson. One of them, Girl in the Middy, an oil sketch of Rosa Breithaupt, herself painting by the water's edge, contained a bonus painting on the obverse side: a landscape catalogued as "unknown" in subject matter. Jackson's fellow member of the Group of Seven, A.J. Casson, actually wrote (in ink!) on it: "The sketch on the reverse side could possibly be an early AY Jackson." That clearly seems to be the case. But what does it depict?