I am going to say its a transmission mount bolt for a 4 speed. I only say that because most autos I have ever seen have studs on the trans and take a nut and rectangle washer to attach the mount. I have never seen a 4 speed with studs but only the bolt here^^^ Xs2 not to say someone can remove the studs from the auto and substitute bolts but I don't think that is the correct way.
