coolant tank cap

[old cruiser]

Just bought the missus a 2006 fiesta, nice little car for her, however, was checking fluids etc today and went to remove the coolant cap and the yellow cap , spring and black plastic part you would normally see on the underside of the cap were all in seperat parts as it were!! it would appear that the black plastic part pushes into cap (with spring and valve) and then twist to secure, at least that's what I've done , anyone else had this happen before?

Cheers Pete