Well, looks like it is an O.S. afterall. It is for an FT160.
http://www.osengines.com/parts/images/46181000.jpg
Listed here at the bottom of the page
http://www.osengines.com/parts/xr36100.html
It is so small though. Here are a couple of pictures of it next to my Saito 120. The throat is half the size. Is that possible?