[Landsvw]

Aha! I think I maybe found the cap that fits this specific bike. It's not a twist in at all. Only straight down to click on the inner ledge. It's a Honda Rebel type cap... Here's some pictures, see if you think I'm on to something. Nothing Chinese seems to be right. All of them are either 1/4 twist or plastic tank. This must have been an earlier metal version to meet street legal requirements or something. Not sure. With the catalytic converter in the pipe I'm assuming something like that. Roketa websites show nothing similar, and after a long combination of search words through eBay came up with these pictures:







And then, mine again...