Nguyen dynasty 'Tu Duc Thong Bao' pattern coins comparison. There are 2 different dies for both pattern specimens. The left side is Brass variety (unpublished), and the right one is Copper variety (BnF# 463). There are several different on reverse: rim, bold of character, legends allocation, and also centre hole, weight and thickness, but size of both are similar. Estimated 6 to 8 specimens known for copper, and 2 to 4 specimens known for Brass variety. Both are uncirculated and extremely rare.