This is a set of thumb studs made in Seki, Japan by Seizo Imai for the Blackjack Mamba and his custom knives. The dimensions of the outer portion of one side is 0.239 x 0.158". The center diameter measures approximately 0.155". It would work for a blade about .165 to .190" thick. Seizo Imai was one Seki, Japan's finest craftsmen and most famous knifemakers of the modern era. His collaboration with Parker spanned nearly two decades. He made knives for Parker, Frost, Browning, Explorer, Compass, Condor, and many others, including his own brands Imax and Seizo Imai. His custom Loveless style knives are works of art. After his passing, several vintage old stock models were found at his warehouse. This is one of these rare finds.