{{m|gem-pro|*finþaną}}

You could make a note, I suppose. Are there any comparable verbs with -nþ- that do have the spirant law?

Maybe not. I don't know. But at least in the two perfectly analogous *bindaną, *windaną the -d- was original. Maybe *finþaną was changed by analogy with these two?

I've added it. What I hadn't noticed before is that there's apparently an attestion for Old Saxon fīthan alongide normal Old Saxon findan, but no such form seems to be attested in any other language. And one never knows if it actually exists or if it's just copied from dictionary to dictionary. But all right, I said the effects were "widely hindered".