I just got one of these 6x17 backs this week, and I too bought it from "oneworld" on ebay. Great service, and it got here in less than a week.
I have posted my first shot in the standard gallery on Friday titled "Good Advice."
It will be used on my Sinar F1. It comes with adapters for 6x9 and 6x12, in addition to the 6x17 format. It is light weight, and straight forward to use. There are two pieces, the film back and the ground glass back. The glass back has abuilt in bellows hood. I still use a dark cloth with it, but it does help.
I was hoping to shoot some today, but it is 5 degrees F with 50-70 mph winds and snow, maybe I'll use the Hassy today.