It looks like this Part has been superseded by LEGO Part 3626b Minifig Head Plain [Blocked Open Stud].
There are three known mold versions of this part:
3626a Minifig Head ... [Solid Stud]
This seems to be the original version, introduced in 1975. The top stud is solid.
3626b Minifig Head ... [Blocked Open Stud]
This seems to be a later version, introduced in 1978. The top stud is open, and blocked.
3626c Minifig Head ... [Hollow Stud]
This seems to be modern version, introduced in 2011. The top stud is hollow.
The mold comparison image shows all three versions.
Click on a colored part below to see the Sets and MOCs that use part 3626a Minifig Head Plain [Solid Stud] 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 3626a - Minifig Head Plain [Solid Stud] in the database, in use between 1975 to 2011. See Molds and other related Parts.