Fixed rod type with bases hardened, precision ground and micro-lapped. All other surfaces are satin chrome finished. The screw is also precision ground and lapped. Ratchet stop. Spanner wrench with each micrometer.
Accuracy:
Instrument error: ±3µm.
Flatness of reference face (bottom face of base):
1.3µm for 60mm/2.5" base.
2µm for 100mm/4" base.