If you brightly illuminate the front of the card (so that the light is diffused and can be seen through the card), can you see the original monster card text between the postcard back and the monster card face?
I had a buddy in the 80s who used to convert card stock art into postcards by adhering a custom postcard backing to a pre-existing card stock art card. It was a fun idea, and he made it sound like it was something that people do and not just a “him thing”,
lol