I know my mother redeemed them every so often otherwise it was lost revenue. When the paper coupons were being phased out she turned in a large quantity she had on hand and these were a few stragglers that were saved and/or overlooked.
This one is not mine, but I thought it would be of interest to the thread. This is a replacement (star) food coupon -- only the second one I have ever seen. These would have been used to replace printing errors as inspectors found them.
Given how many food stamps were printed there ought to be many more of these, but no one was looking for them at the time so they flew under the radar. Only now that eBay has reversed the sales limitation on these are they starting to gain collector interest. I suspect more will slowly creep out of the woodwork.
The thing I like about these is that there are only six designs in the entire series, so completing a type set is relatively easy and not too expensive. And if you want to specialize, there are many different series years to collect.
Here's a complete set (not mine, though I'm working on one).