From the OPC webpage:
The Presbyterian Guardian (1935-1979) was an important voice in the early years of the Orthodox Presbyterian Church in its vigorous opposition to modernism and its proclamation and defense of Reformed orthodoxy. Established on the eve of the founding of the denomination, it was closely associated with the OPC, although it remained an independent magazine.
Editors of the Guardian during its remarkable 43-year publishing history included H. McAllister Griffiths, J. Gresham Machen, Ned B. Stonehouse, Charles J. Woodbridge, Paul Woolley, Leslie W. Sloat, Robert E. Nicholas, John J. Mitchell, and J. Cameron Fraser.
Can be found (archived) here.