What is a Lefever Nitro Special 12 gauge double barrel single trigger sn286651 worth and when was it made?

I also have a Lefever Nitro Special 12 guage, serial #337... which was made between 1936 and 1937, and it's also a single trigger (which I think is rare). Yours was made around 1929. While these are not expensive guns, they are considered high grade American made shot guns and they still shoot great to this day. Lefever made some of America's most collected shotguns as did the Ithaca Gun Company which made your gun. But the double guns made by the Lefever Arms Company are much more rare and much more valuable. They can sell for thousands, and even hundreds of thousands for some of the rarest. The Ithaca Gun Company made about 236,000 Nitro Specials between 1921 and 1948 after they acquired the Lefever name. They came in different gauges but the most common was the 12 gauge with a double trigger. If it were a 16 gauge, it would be worth about 20% more. I'm not sure how much the single trigger adds if anything. Mine is in good to very good condition and I think it's worth around $450. The fact that yours is older should not make much of a difference. The determining factor is the condition. If you do a little research on the internet you can find out everything you need to know about determining the condition and value of your gun. That's what I did after I inherited mine and I've been an admirer of collectable side by sides ever since.