It's called a "ring gag". I found a couple of movies at least where it's used, there are more but I cant remember which.
Sadly in both of these they take the gag off after a couple of minutes.
REAL-389 (Shizuka Kanno), fifth scene
REAL-480 (Miki Sunohara), last scene
edit: one more. REAL-447 (Yuna Shiina)
There is another great scene in night24 268, where they put on a dental gag which works pretty much the same way and have the actress keep it through the whole scene.
It's also uncensored.
Hope these were new to you.
Also which JAV is that screenshot from? I'd love to find more movies featuring these as well.
