I thought I was being slick years ago when the #3 exhaust valve broke and I replaced all the mushroom valves in my 9N with the newer valves for an 8N. Seems Ford discovered the same thing. The 9N6505 valve suddenly disappears and the 8N6505 valve makes an appearance. My OSI catalog does not show any 9N valve parts.
Interesting, I would recommend keeping the old 9N valve springs as they are stronger than the 8N springs. You also need two special tools to hold the adjustable lifter still while you adjust it.