They were marketed by Fleetwood (I'm pretty sure) when I bought a few in the 1980s & 1990s. You could purchase them with a folder to mount the stamp in. The folder contained additional information about the birds depicted and the artist.
I can't find any of the folders at the moment.
They also marketed a set of stamps depicting a bird associated with each state. Here's the Alaska version:

I put mine in Vario stock pages after my migratory bird revenue stamps.
Robert