Norbar torque wrench calibration

[tscalfa]

I've got a Norbar torque wrench that I would like to calibrate and adjust if required. My other torque wrench has hex socket screw for adjusting the calibration on the Norbar one pictured I can't see any obvious way to adjust it. Does anyone know how to adjust the calibration on Norbar torque wrenches? I have included a couple of photos of my torque wrench.