my memory of the yellow packs, which were the only ones I ever saw or bought, is somewhere in late spring/early summer '74, between buying series 7 and series 8
it was a family trip to Pennsylvania, possibly Memorial weekend. My brothers and I had only been hardcore into wackys for a short time, a few series, starting with series 5 in late '73. At this point we were always on the prowl for wackys, looking for new series. And when we saw these at the store, we were floored. My parents groaned, but they bought some anyway.
One thing I did as soon as I got back home was to put the sticker onto the poster, to have the images side-by-side and see the differences.
I had 4 posters, 3 of which I put stickers onto - Jail-O (used a wonder bread sticker), Blast Blew Ribbon and Burpsi. The 4th was Toadal, which I never did find a sticker for...
My guess is that Norm was off painting these during the series 5-7 timeframe which cut into his production and is one of the reasons there are fewer Norm pieces through those sets.