It looks like this Part has been superseded by LEGO Part 3001 Brick 2 x 4.
Merged - Brick 2 x 4 [No Cross Supports] Transparent with Frosted Horizontal Line [image forthcoming]
Click on a colored part below to see the Sets and MOCs that use part 3001a Brick 2 x 4 without Cross Supports 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.
There are 4 Mold variations of part 3001a - Brick 2 x 4 without Cross Supports in the database, in use between 1954 to 2006. See Molds and other related Parts.