Kodak, Tiffen, lots of folks made 'em. Kodak probably defined the 'Series'. You can find them on ebay easily enough. There used to be a local camera shop that had a big box full of retaining rings, old filters, and such. Maybe you can find a place like that. Kodak and Tiffen made good ones, I had a Harrison though that was just awful, wouldn't fit properly no matter what I did.
Now, what you need to know is what metric size fits that thread so you can get an a step up ring to thread into the Series VI filter holder. I THINK it's 43mm, but you check around the web and verify that. If so, then you want to get a 43->49mm adapter so you can use standard 49mm filters, for instance.
Nathan