Unknown Bolt (Help)

I have researched and asked numerous people about the origin of this bolt (I have about 20 of them.) It is 3 inches long and the 6 sided head is 6/10" in diameter. Under the head it has angle (countersink) and the threads change diameter and get smaller about 2/3 the way down to the end of the bolt. The bolt is steel. History: The estate sale I bought it from had a gentleman who owned a bait shop. In his back room he hand-made fly fishing rods out of bamboo, and he constructed small fishing boats. He was very eclectic in that he was a writer as well as he had a degree in music! I cannot figure out what this could have been used for and again....there's no numbers or letters on any of them.