I got these 'bolts' or tangs from FineWatchBuckles in Germany who uses these bolts with their buckles. The bolts are from Maddog. I have to say that the bolts themselves are a work of art. They are just awesome.
The bolts are made of steel, brass, bronze and or copper. Some of the bolts have been given special treatments like Mokume-gane and Damask steel.
A bit about Mokume-gane. It is a mixed-metal laminate with very distinct layers in its construction. It is Japanese (?) for burl metal and it is borrowed from one form of forging swords or edged weapons.
Damask steel or damascus steel is a type of steel used in sword making. It is created from wootz steel and it is characterized by very distinctive patterns of banding and mottling, much like flowing water.
You can see the lovely pattern or layering on the bottom of buckle and the side of the bolt.
The bolts themselves are pretty large and comes in at 22mm in length, 4 mm wide and at the thickest point, 5 mm thick. These are not your common garden variety bolts (or tangs). To use these bolts, you will need pretty large holes on your strap. They also require larger then normal spring-bars. I like them.