What it appears you have is a late 18th century nail. I really like them. NOW, you need to get some microcrystalline wax and a steel container and propane torch. Put the wax in the steel container (outside), and the nail in the wax. Start to heat SLOWLY until it melts. Don't do it on the deck, it can catch on fire, so have some water or fire extinguisher on hand. The wax will melt completely. THEN air bubbles will start to come out of your spike. When the bubbles start to greatly diminish turn off the torch, get some tongs and grab the nail out of the wax and put on a paper towel to blot off the excess wax. If you don't do that, in 6 months you'll have a pile of rust.
I frequently find similar nails on 1750s French & Indian War encampments and other 18th century sites. Here are pictures from two F & I War sites showing these type of nails, They are Wagon Strake Spikes and were used to secure the iron wheel rim to the wooden wheel.