The inverted arch has the same curve as the regular arch 2339. Unfortunately, this results in a gap between the bottom edge of the curve and connected plates/tiles as shown in the photo.
Two variations of this part exist (refer to the photo below):
First type does not allow inserting a hollow stud in the corresponding tube.
Has anyone ever noticed all these pictures?
Click on a colored part below to see the Sets and MOCs that use part 30099 Brick Arch 1 x 5 x 4 Inverted in that color. You can find an overview of all LEGO Colors here.
This part has been used by MOCs in the following colors but not officially released.