I agree w/Paulbe. I used the 12585 back when I had a DR Summicron (I currently have a Rigid of the same vintage) & greatly preferred to use a simple vented screw-in 39mm hood. IMHO, the 12585 was kind of a PITA to use. I do have a bias against clip-on hoods, however, as they never feel entirely secure to me when clipped on. More importantly, though, I didn't find the 12585 to provide any more protection against flare than a screw-in hood despite its significantly greater size. The only neat thing about the 12585 is that it can be reversed on the lens for storage, but since I usually just leave hoods on my lenses & don't use everready cases, that's a pretty useless feature for me.
FWIW, I don't what brand of filter you're using, but I had no problems clipping the 12585 over regular, not extra-slim, B+W & Heliopan 39mm filters.
As far as heavystare & American Eagle (another vendor on eBay who sells similar goods), I've had great experiences w/both of them.