If this was a factory installed radio, perhaps the factory kept the information and might release it to a confirmed owner of record (remember this was an anti-theft measure in its day). I know the VW dealer was able to to contact VWOA and provide the unlock code for my wife's Passat when the battery decided to suddenly retire from duty. If this is an aftermarket radio, not sure who might be able to unlock it. As an end run around the lock, there are some companies that will convert vintage radios with modern digital tuners and amps inside the vintage case. They look original but benefit from a brain transplant. Not cheap, but if the vintage look is what you are after, it is one option.