It looks like this Part has been superseded by LEGO Part 15254 Brick Arch 1 x 6 x 2 - Thin Top without Reinforced Underside [New Version].
Click on a colored part below to see the Sets and MOCs that use part 3307 Brick Arch 1 x 6 x 2 - Thick Top 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.
This part has been used by MOCs in the following colors but not officially released.
There are 3 Mold variations of part 3307 - Brick Arch 1 x 6 x 2 - Thick Top with Reinforced Underside in the database, in use between 1971 to 2017. See Molds and other related Parts.