In the 1980's and 1990's, baseball cards being distributed with food products was quite common. Cereal, bread, soft drinks and snack foods of all kinds ran promotions containing sports cards. As you work backwards from there, baseball cards being distributed with some product becomes more and more common. Though I am not clear on the exact method of distribution, a card set associated with food items that you may not be aware of is the 1913 Voskamp's Coffee Pirates, a regional 20-card set released by B.H. Voskamp's Sons Inc Wholesale Grocers.
It may be difficult to tell from the photo, but each card features a black/white posed image over an off-white background. The back contains the checklist for the 20-card set and the following note:
"When you have a complete set bring them or send them to us. We will return the photos to you and furnish you with one reserved base ball seat, Grand Stand, Forbes Field, or two general admission seats, or four bleacher seats."Below that note is the B.H. Voskamp's name and their address in Pittsburgh, PA. There are surely earlier examples of this type of promotion using some kind of collectible set, but I can't name it right off of the top of my head. The formula was pretty simple: Entice people to buy your product and collect some cards, and then trade the complete set of those cards for some type of goods or service. It was also common to make the task of actually completing the set particularly difficult by including a 'chase' card.
In the case of this set, the chase card must be that of Ed Mensor, as the card has simply never been confirmed to actually exist. Hank Robinson also used to be considered 'unconfirmed' until recently, meaning the population of that card is exactly one.
Considering the cards were to be returned upon set completion, one would expect that even if rare, a few Mensor and Robinson cards would have survived. Surely more than just one Robinson, at least. Of course the set is over 100 years old, and it was only a regional promotion, so the quantities of the entire set is certainly much lower than any national promotion or collectible card of the time.
Prior to the internet, there probably weren't too many people outside of the Pittsburgh area with any experience with Voskamp's. Even with the internet, finding examples of this set isn't the easiest thing to do, though a few will sell at major auctions, and even on eBay, every year. Just how rare is this set, though?
Realizing the flaws in relying on SGC and PSA population reports (not every card has been graded, how reliably does either company remove cards from their reports when they are re-submitted or crossed-over, etc), consider this: To date, PSA and SGC have graded a combined total of 79 1913 Voskamp's Coffee Pirates cards (of those 79, 8 are Marty O'Toole). By contrast, the two grading companies have graded a combined 44 T206 Honus Wagner cards. In other words, for every 2 Voskamp's that have been graded, 1 Honus Wagner T206 has been!
Now, I think it's safe to assume that the T206 Wagner is probably more likely to be graded than a Voskamp's Coffee card. But even if only 25% of Voskamp's are graded and 80% of Wagners are, then even the most common 1913 Voskamp's Pirate has a smaller population than a T206 Honus Wagner!