Recently I bought a 12 ga Superposed Lightning salt gun(1968). It had just come back from Browning where it had been rebuilt for the original owner. All the receipts and paperwork were in the case and proper. The gun is unfired since repair. I paid what I thought was a fair price for the gun considering it was a close to a new Superposed as I'm likely to ever get(Less than the cost of repair).
My question is :What does this remanufacture do to the value of a Browning when it is done by Browning and is documented?
To answer your question, even if done by Browning and documented, the value is considerably decreased and a hard sell. Sinec you have a Grade 1 and it is not a collectors item, no problem, just use it. Even higher grades like Diana and Midas are devalued and a hard sell. I have been offered a few and will not purchase not because of the Salt stigma but just because it is not an original gun.
In terms of the AGNO3, I have crystals and have made up the solution but after 40 years if it was a Salt gun it would have shown signs of damage. I have purchased and sold Salt Wood era guns from/to dealers including a Pigeon Superlite with no problems whatever. I certainly would purchase a 12 gauge Magnum from that era if a decent gun. I have a 1963 Magnum and it is a great Goose gun at F/F and 39 thousands constriction but that was just the luck of the find.
Good luck with your Super.-Dick