1989 Historic Limited Editions Don Mattingly Postcards (Series 1) Baseball Forum

There are prototype versions of the '89 and '91 Mattingly postcard sets that feature a gold stamp on the front.  Here is an example of one:

https://mavin.io/item/1991-Historic-L.-E.-Series-2-Prototype-Don-Mattingly-%2310-Yankees-PSA-9-Mint?itemId=202707702598&q=mattingly%20prototype

From looking at other prototype sets in the database, it seems like they should just be added as separate sets?  I don't have any background info on how these were distributed.