It has been a while since I was inside the S310 I have at home, but a short pot with a hexagonal end and a grub screw across the inlet hole is the correct item in one of these. The pot design was simplified and lengthened when the 400 series were released.
Unfortunately I cannot remember how far the S310 pot sits proud of the face though. I do know the TP screw is screwed in a fair way though.
People who have been there focus on the fundamentals. People who sit at keyboards all day focus on the trivial and inane.