I suspect they went from 2 and 1/4 film to 35 mm, which led to those mid 70's awful quality photos - though I like the composition of the 73's.
I agree with the '69 set being terrible in that it largely copied photos from the year before or even much earlier. That combined with all the "black caps" makes the 69 set really pointless if you have the 67 and 68 sets. There's maybe a hundred 1969 cards that are even remotely interesting.
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