I grew up in RI, and I did send away for some of the SI posters of some Red Sox players. But I also had 3 of these cards that I had cut out myself.
I know I had the Lou Brock, Juan Marichal, & a 3rd.

I know I had 3 cards that were on cardboard stock and blank backed. I also know I cut these out myself. I also know they were the exact same photos as the SI posters.

I thought they came from a cereal box, but I can not be sure. Unfortunately I believe I sold them to a local dealer in the 80's along with other 1960's Topps cards as a batch however. But I might have kept the Lou Brock. I'll have to see if I still have it.

But facts I remember:
1) Cardboard stock, blanked backed (gray back I believe)
2) I cut them out myself
3) I had 3

And no, I've never eating any cereal from India. But if it was a local test promotion, I would have gotten them in RI.

The 1969 Nabisco set used a lot of the same pictures as the SI posters, so if your cards didn't have the SI numbers on them, you might be thinking of those...

kudos Doug.

Just pulled out the ad posters, and it may be noteworthy that the 9A1 "Catfish Monday" (Catfish Huner? Rick Monday?) and 9A2 "Campy Campernaris" (Bert Campernaris?) are listed as future for sales posters but the players pictures are not shown. Blank spaces are shown for those players. If those players (sic) are in the card set, they didn't come from this poster.

Yes, you are correct. My cards did not have the numbers on them and I only had 3, two I know for a fact had same photo as SI poster. I must have had the Nabisco.

Wow...that may explain those blank football "cards" that PSA slabbed. It seems much more evidence is pointing to these being cut from SI ad pieces over some Indian corn flake cereal box.