Black door posts have indention's on all four at top

[Cgo]

Mine have them. To me, it looks like where the guide nubs are. I'm pretty sure these pieces are plastic so most likely manufacturing process or from the pressure from the nub attachment points. Some have them and others dont, so, probably depends on the production batch variation.

 

[Toddasaurus]

Mine have them. To me, it looks like where the guide nubs are. I'm pretty sure these pieces are plastic so most likely manufacturing process or from the pressure from the nub attachment points. Some have them and others dont, so, probably depends on the production batch variation.

This.