IPC-7351C polarization mark in real-life?
One example is a card I recently design.



As you can see U1 follows this convention which is capture in  IPC-7351C

Likewise here is a render of a card I have in fabrication+assembly (2more weeks and you would see a picture and not a render)



As you can see, U2, U8, U7,U3 ... follow this convention.