This unit was originally manufactured by Micro Medical Ltd. in Kent.
Unless it has been damaged, it should not need calibration as such - but rather a calibration check and a saving of the "cal" factors if within spec.
But yes, you will need a 3-litre calibration *
syringe.

*
The original Micro Medical part number for the syringe was SMI2125.If you want to get really technical, there is (or at least,
was) a Pressure Calibration Kit as well (part number CAL6000).