NC HT1.0 Original Instructions Manual page 28

TIGHTENING TORQUES FOR GENERAL USE SCREWS, BOLTS AND NUTS.
Screw and bolt material grades are shown by numbers punched on the screw and bolt heads.
Prior to tightening, be sure to check out the numbers as shown below.
Punched number
None or 4 Standard screw and bolt SS41, S20C
7 Special screw and bolt S43C, S48C ( Refined)
Kubota Torque Settings
Screw and bolt material grade
28