It was the date of the film, the 1970's pics are lifted from the movie
I thought it was filmed in 1969 - maybe a Saga moment. For on-track shots, they had a reel-to-reel camera mounted on top of the car (in fairing). Trouble was, a reel of film didn't last as long as a tank of fuel, so the car had to make more pit-stops. They were using the very last pit, and you got a grandstand view of the pit activity from the top of the old ACO tower.
Nice set of photos, though I'd question the description against the 'Pit Straight 1970' (17 pics from the end). 'No Garages'.....WTF. They may have been smaller than todays, but there were still 55 garages!!
Pity, I lost a heap of photos from '68 '69 and '70 in a house move years ago.
Del