I read some of my father's Autosprints. This is what I discovered:
-the two sessions qualifying format started in 1980 by Goodyear request, as they couldn't afford bringing qualifying tires for four sessions;
-in 1979 all four timed sessions counted for pole position;
-from 1976 to 1978 of the four timed sessions, three were qualifying sessions (two on Friday and one on Saturday afternoon); on Saturday morning it was held a free practice session;
-from ??? to 1975 four sessions as in 1979
-In 1979 Autosprint reports that the two qualifying session format was used "some years ago" but the number of practices were doubled to make more entertaining the race weekend. I still can't find which year(s) they are talking about.
I assume that in 1980 the sessions were one hour each, but I'm not sure yet.