Here's a possible explanation that I've got about them which ties into your Autosales theory...
One of the products that Autosales Gum put out was Dentyne. I found a picture of some wrappers that are for sale on eBay which the seller dates to the 1920's.
Those "Colgans proofs" sure look like they would fit inside those wrappers, don't they?
Since they have no advertising on the back, it's hard to know if they were actually inserted into any sort of packaging, or if they were merely proofs of some sort based solely on their limited overall numbers.
It would be nice to be able to find any type of period Dentyne advertising to see if they mention these at all.