RS Mach gas cap?

[4Aaron GE]

So there's a RS Mach gas cap that exists. Says it's for PP1, but it's not the normal screw on clicky type cap, but one of the older style caps you see on motorcycles sometimes. Did some cars come with the older style caps for the filler neck or something? What gives?

http://i.imgur.com/aucyM0w.jpg
http://i.imgur.com/ZxW648D.jpg

[bigB]

looks to be an oil cap from under the rear hatch

[4Aaron GE]

That makes so much more sense! What do you suppose these are worth?