These Post Cards were not a failed experiment, and were not sold in packs with gum. They were given to dealers as an incentive, to be sold over the counter. The set total is 24.
I just picked up an R189 Tintype type card and it got me wondering why they are classified as an R card. Is it simply because they were issued by Gum Inc? I believe that R189 was the last R number issued by Jefferson Burdick. I did a quick search here and stumbled on this thread by uber collector Connecticut Yankee (who I know is very well respected). I guess my question is: how do we know that these were given as an incentive to dealers? Is there a reference to these somewhere that I don't know about? Okay, that's a rhetorical question, but if anybody knows anything about these I'd be curious to know.
I'll post the card once I receive it.