The issue of the ownership of a gun lost so long is open to debate. The law of "finders" varies from state to state. As most here have posted the rule usually is that if the original owner is known then the property still belongs to them. But, as also noted, there are statutes of limitations and other possible issues.
I agree with the general sentiment here that the gun belongs in a collection/museum but I doubt the present owner would willingly "give" the gun away even if it is the right thing to do. I think all of us know it is rather valuable.
And, who knows the gun's history since that fateful day when Nash and friends probably had a drink or two in the marsh and left the gun when they drove off. The gun may have gone through any number of possessors and/or sales since then.
If the gun is out there, possibly it could be purchased at a fair price and put in a museum. No recriminations necessary. The talk of STOLEN etc. will just drive it further underground.
Just a thought.
Jake

R. Craig Clark
jakearoo(at)cox.net