The plaque could be make by splicing a Cave Background Seed and a Door Seed during the weekend it was available. The recipe was replaced with the Happy Joy Plaque which was released directly after the Rollback Plaque was discontinued.
Rollback Plaques were originally released as "perma-items," which meant it returned directly to the player's inventory when broken. After the item became unsplicable, it was changed to never drop anything when broken.
When the Rollback Plaque was released, the item's seed was available as a random item from the Small Seed Pack. This was an oversight which was patched shortly after it was discovered, but any seeds which were given out were not removed, making the Rollback Plaque Seed a highly valued collectable.
This items has two parts or items from one even that is almost the same.