That's not a standard bolt. I think it is something that we have made. We call it a "headed pin", "transducer pin" or even "pivot pin". It is internally tapped to except a 1/4-20 bolt when it is installed. You may see if someone has an extra package of hardware that they can send you or just the headed pin and bolt with washers from an old or damaged transducer. If you use a standard bolt, be sure not to tighten it too tight or the transducer will not rotate up when you hit debris in the water.