What the 60 A rating means for your circuit
The 399700103: The 4-position sibling 0399700104 carries the same 60 A, 300 V, 6 AWG, and pitch specs — the only functional delta is one additional circuit per level. If your panel was laid out around the 4-position part and you're evaluating the 3-position as a drop-in, the mounting footprint is identical on pitch and the horizontal orientation is the same, but the wiring will be one circuit short. Confirm the wire count delta is acceptable before swapping.
Plating, termination style, and field service
The contact is copper with tin plating; the clamp is brass with nickel plating; the screw is bronze with tin plating. Tin over copper on the contact face gives good solderability and adequate oxidation resistance for a power block that isn't being mated and unmated frequently. The rising cage clamp design means you insert a stripped wire and the cage rises as you drive the screw — it grips the full cross-section of a 6 AWG conductor without cold flow, and the clamping force is set by torque spec, not by feel. Field wire replacement is straightforward: drive the screw, insert fresh wire, torque to 1.36 Nm.