There are four known versions of this part:
3940a (First) Support 2 x 2 x 2 Stand with Blocked Center Hole
This is the original version, introduced in 1979, and discontinued in 1988. It has a blocked center hole.
3940a (Second) Support 2 x 2 x 2 Stand with Blocked Center Hole
This is an updated version. It has a blocked center hole with stud holders on the bottom.
3940b Support 2 x 2 x 2 Stand with Reach-Through Center Hole
This is an updated version, introduced in 1988, and discontinued in 2012. It has a reach-through center hole.
19798 Support 2 x 2 x 2 Stand with Reinforced Underside
This is the modern version, introduced in 2012. It has a reinforced underside.
The image above shows all four versions. Note that you are looking at the bottom of the parts. If unsure, use the modern version 19798.
Click on a colored part below to see the Sets and MOCs that use part 19798 Support 2 x 2 x 2 Stand with Reinforced Underside 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.