My friend called me about an hour ago and said the ingot is now online for pre-bidding at the auction house, which specializes in rare silver and gold ingots and bars. One of the best at that I hear, which is why my friend was very excited when they called him immediately on receiving pictures of his ingot and told him. "You've got a VERY NICE item here!" They then paid for the shipping and special insurance via their own set up accounting for that, without any obligations, just so they could research it. With the instant phone call after seeing pictures and offering to insure/ship/research it with no strings attached, that's when we suspected he might have something special here. Of course none of that means this ingot is going to go for any kind of huge amount of money, but then again one never knows...
Notice the live in house and online bidding is one day only later this month. Also, notice they say "An exciting piece that will undoubtedly reward further research." That should intrigue some people as it adds to the mystery.
Also, as with coins, I bet it's the rarity of a bar or ingot (how many there are) and not necessarily the historical significance that drives the price up. Just like a coin, the less there are the more the value due to rarity. Since none of us can find any examples of this ingot online, I think it might go for a really good buck.
And just the same, it does appear it might also have some important historical significance being less than 50 miles from Sutter's Mill, and the remarks about the area where the mine was located being where "Herbert Hoover started his mining and assaying career in this area, known as the center of the Mother Lode." Further research might prove even more importance as to it's historical value I would suspect.
http://stacksbowers.com/auctions/auctionlot.aspx?lotid=435911