Brown Sharpe Mill/Screw Machine Lathe Circular Tool Bit Post Holder 135-100-210
For mounting on a cross slide.
This one has a modified circular block - instead of holding the usual circular cutter bit, it is installed with a removable clamp that holds 2 small square bits for cutting spaced grooves into round stock.
It can hold one 7/16" max square bit or the existing two narrower graduated square bits.
T bolt clamp threads are good.