There are three, perhaphs even four, known mold versions of this part:
3245a Brick 1 x 2 x 2 with Center Pin
This version has not been used in any set, so we are not sure if it actually exists. It should have a single bottom pin similar to 3004 Brick 1 x 2.
3245b Brick 1 x 2 x 2 with Inside Axle Holder
This is the original version, introduced in 1991. The inside has four large, asymmetric ridges to enable insertion of an axle.
3245c Brick 1 x 2 x 2 with Inside Stud Holder
This is the modern version, introduced in 2010. The inside has four small ridges to enable insertion of a single stud. The ridges can be symmetric or asymmetric.
772 Brick 1 x 2 x 2 with Hollow Underside
This is the special version, used between 2006 and 2008. The underside is completely hollow.
The mold comparison image shows 3245b and both 3245c versions. If unsure, use the modern version 3245c.
Click on a colored part below to see the Sets and MOCs that use part 3245c Brick 1 x 2 x 2 with Inside Stud Holder 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 4 Mold variations of part 3245c - Brick 1 x 2 x 2 with Inside Stud Holder in the database, in use between 2010 to 2019. See Molds and other related Parts.