Postage Due stamps?
When I was a kid (1970s), the post office staff would sometimes have the old postage due stamps (issued in 1959, I believe). As a child collector, they would begrudgingly sell some to me for the price indicated that was due. As these were being phased out already at that time, so I found them tough to get.
I presume these are basically useless except possibly to a collector at this point?
[Stolen web image to show what I am talking about.]