I believe it is a Motorcycle Larry part. If you do a Google Search for the phrase "st1300 ram ball mount", then click on 'images' (rather than the default 'all'), you will see a number of identical parts that come from the Motorcycle Larry website.
Here is a direct link showing that part (scroll down once you get to the page):
ST1300 RAM BALL MOUNT COVER
I don't think that Motorcycle Larry is still in business.
Michael