Given the size of these craters, I assume they weren't discovered until the space age. Perhaps during a Mariner flyby? When were these craters first discovered?
The spacecraft also provided the first closeup pictures of the two small, irregular Martian moons: Phobos and Deimos.
This must have happened somewhere between November 14th, 1971 and October 27th, 1972. Here is a picture from January 27th, 1972 which shows them:
FWIW, the only other spacecraft to reach Mars between then and 1973 was the Russian Mars 2.