Folding Knife For ID Please

[trenchwarfare]

I've had this knife for some time now. It was listed on ebay, as Japanese. The blade is 5" long, concave, with a groove on one side. Overall length is 13". The ring "stopper" on the tang reminds me of Chinese opium knives. The groove for the blade goes almost all the way to the end of the handle. Blade was probably longer. Shows lots of long, hard use. Anyone know where this is from? .