B64 Type 3 steering arms are a slight tweak on the geometry of our Type 1 blocks to gain more stability on tracks with larger sweeping turns. The Type 3 mimic the shorter leverage and quick reaction of Type 1, but with a more mellow Ackermann that is suited for flowing tracks when you need less steering and to stay smooth. The holes are a replica location of adapting the B6 steering arms onto a B64, while still keeping the kit steering stops.
You may ask...what about Type 2? Those are still in development, not ready for prime-time yet!