If it's any consolation, I never noticed it either when watching endless reruns on TV over the deacdes. I didn't even know about Doohan's "deleted" digit until maybe 10 years ago or more recent. I finally noticed the shot in question when either I read someone else's obsevation on the matter (and then popped to TrekCore to see for myself) or when I was reading a page at Nyrath's "Atomic Rockets" site. Honestly, I'm not sure which. If the latter, I was reading Nyrath's entries about personal gear. I reach a section about pocket sized "tools" ans Nyrath presented examples like the Doctor's sonic screwdriver, Gary Seven's "servo", and the tuning fork inspired gizmos from "Fantastic Journey" and the TV version of "Logan's Run". Nyrath also presented a series of frames of SCotty's "magnetic wrench". Having finally heard about Doohan's finger by this time, I noticed one particular frame and realized, "Whoa! There it is! Or rather, isn't!"
Now, I understand Doohan tried "hide" the injury, either for personal reasons or possibly at the request of the production (I'm not sure which). But looking back, I kinda' wish he hadn't. Something like that could have actually worked in the character's favor, a "hint" of his past. Oh, no real need to draw attention to it like any on camera exposition or backstory, just don't make an active effort to hide it. I rather like the notion Scotty suffered an occupational accident when he younger and not as experienced or cautious.
Sincerely,
Bill