Well, I too was unable to find specific information on the company, but looking at listings for Yankoware items that are currently up for sale, I was able to determine a few things. It is Japanese company. All the listings I saw said "Japan" and not "Nippon". Using the name Japan would be post-1921. All the listings I saw said the pieces dated to the 1930's or 1940's. So that at least may help to date your piece. I would guess that they were not exporting to the US during WWII and may have gone out of business at that time. But that is conjecture on my part. Hopes this helps to start the ball rolling. Photos would be a big help though.