This 5/8" diameter steel bolt has been hollowed out, and has a threaded, removable head.  During the "Cold War", devices like these were used to transfer messages and microfilms between two individuals without the need for them to meet.  The bolt was left in an inconspicuous location, such as on a bridge or automobile where it would be replaced with an original bolt.

We are now offering our hollow bolt in type 304 (18/8) Stainless Steel.  These bolts have a nice matte finish, and will withstand moisture and mildly corrosive atmospheres.  The extra cost for this item reflects the added difficulty in machining.  $48.00