It looks like this Part has been superseded by LEGO Part 3068b Tile 2 x 2 with Groove.
Click on a colored part below to see the Sets and MOCs that use part 3068a Tile 2 x 2 without Groove in that color. You can find an overview of all LEGO Colors here.
If a color is missing, it probably needs to be added to a Set Inventory.
This part has been used by MOCs in the following colors but not officially released.
There are 3 Mold variations of part 3068a - Tile 2 x 2 without Groove in the database, in use between 1965 to 1977. See Molds and other related Parts.