That rifle could very well be a 1st, or 2nd year production and like what was already mentioned any pre '98 brings a premium. It's been around the block a few times, but from what I can see it's not in bad shape. We should all look that good at 120 years old. From what I was able to see I think you even still have the elevator with your buckhorn rear sight. Lots of times they're missing. I would've liked to have seen the front sight and entire butt as well, but that letter from Cody is very important in order to determine the value.
Museums use oooo steel wool and oil to clean off rust and grit without removing the finish from items like guns and swords, but I wouldn't be aggressive with the cleaning at this time. Believe it, or not oil is not a preservative, but a lubricant. I've seen rust develop in areas where there's been an oil build up. Wipe it down with a moderately oiled rag to remove any gunk. Then wipe down real good with a soft clean cloth before finally giving it a going over with a silicon cloth and send a cleaning rod down that barrel.
Worry about value when you have all it's info. Those 92's are great and you're very lucky to have gotten one as old as that, cause those aren't seen every day. If it's ever shot it'll almost feel like shooting a .22. Since the bore is narrow the barrel will be heavier than other calibres thus reducing recoil as well. If you have it lettered please let me know and if possible post a full length photo. I wish it was mine.