Whether you already own a 300 series vise or are selecting components to build your own custom work holding system, our Base Mounts will add an extra dimension of productivity to your next project! Compatible with the 201 PV Jr, 300 Standard Base and 305 Low-Profile Base. All Base Mounts have three holes designed for 1/4-20 screws evenly spaced on a 4 5/16" (109.5mm) bolt circle.