If you have a LBS nearby, they should have an assortment of derailleur mounting bolts. If that's not an option, I have an old POS Campy Gran Turismo RD, and my guess is that the bolt will fit your Rally.

I'm concerned, though, that the bolt hole threads on your dropout hanger may have stripped threads from someone trying to force the wrong thread bolt.

If you'd like the Gran Turismo RD bolt, PM me with your mailing address.