Bottom stud holder may be one of the following types:
Click on a colored part below to see the Sets and MOCs that use part 60592 Window 1 x 2 x 2 Flat Front 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.