I remembered another possible appearance of a side ring in a drawing by Hans Holbein, c. 1521. Here the ring seems to appear on a different type of hilt from the other examples so far - I've included a photo of a hilt from Hermann Historica which I think is similar to what is shown.
https://commons.wikimedia.org/wiki/F...he_Younger.jpg
Also here is a somewhat mysterious sword with a side-ring in the Hermitage. The museum dates it to 1400-50, but it's not clear what that is based on... a pommel could have been helpful for dating, but I'd still be inclined to a somewhat later dating. No information on provenance on the website, but it probably came to the museum via the Tsarskoye Selo Arsenal in 1919.
http://www.hermitagemuseum.org/wps/p...661316/?lng=en