I've read, and I could be wrong, but apparently in 84 or 85 Ford started marking owner serviceable parts with yellow. This is why the oil filler caps, power steering caps coolant overflow etc have yellow letters. That may explain why the early ones are plain and the service parts have the yellow sticker. I have an NOS 81 Escort cap that is plain black.
On another note, the flat caps appear to be correct for the 84 and the thicker cap Greg shows appears to be correct for the 85-86