for proof of purchase XX years ago, if you bought direct from a manufacturer, they might have info on the purchase if you pay them to pull the serial.
- not suggesting this, but to get around the BS of not having paperwork, if you know someone on the mainland, they could write you up an invoice for the firearm, that is your proof of ownership. you could even register it in your state first for further paperwork. Even better if they're a FFL. Probably not exactly correct legally .