The province transferred a parcel of land (bounded by Ninth Line, Winston Churchill Blvd., Dundas Street, and the 401) from Halton to Peel on Jan. 1, 1974, when these two counties were created as regional municipalities. This was partly to compensate the contemporaneously-created City of Mississauga for the land the former Town of Mississauga was losing to the new City of Brampton (everything between Steeles Avenue and the hydro corridor to the south... transferring, for instance, Churchville from Mississauga to Brampton). If I'm not mistaken, what's now called Eglinton Avenue in Peel was then still called Base Line Road, and it was renamed, sometime in the 1970s, Eglinton Avenue in keeping with the Metro road it connected to. The towns of Streetsville and Port Credit were amalgamated into the City of Mississauga on the same day.
The stretch of the road west of Ninth Line was, and remains, called Lower Base Line Road, to differentiate it from Upper Base Line Road--which, ironically, is south of Lower Base Line Road, and would be the course of the QEW if it didn't turn southward in the vicinity of Ninth Line (where the Ford plant is).