I found a bike at my granddads

[Poacher]

Disraeli gears confirms @biggs682 's early 60s introduction of the Campag sportman (sic) rear derailleur, albeit with a degree of confusion!
Worth a read, anyway; every little helps with regard to dating that lovely Viking.

 

[Kempstonian]

I'd start by cleaning it up. You may be surprised to see what a difference that could make! You won't devalue it if you are careful. I don't know how familiar you are with bikes but a strip down, clean and reassemble job is what I would do. People on here will always help with advice if needed.