From the above link:

    Sir Adam Beck #1 ten 25 Hz generating units were converted 
    to 60 Hz or modified as follows:
    
    Units 9 and 10 to 60 Hz in 1956
    Unit 3 to 60 Hz in 1970
    Unit 4 to 60 Hz in 1984
    Unit 5 to 60 Hz in 1985
    Unit 8 to 60 Hz in 1990
    Unit 6 to 60 Hz in 1996
    Unit 7 to 60 Hz in 2009

2009! Talk about legacy support.

* https://en.wikipedia.org/wiki/Sir_Adam_Beck_Hydroelectric_Ge...

* https://www.opg.com/stories/opgs-sir-adam-beck-i-hydro-stati...