Mk V Altimeter adjustment

I am in the process of deciding whether to create an adjustable animated altimeter dial for my FSX F2B. The Bristol used a Zenith Mk V.

I am unsure as to how the adjustment functioned in these early aircraft. I gather the altimeter dial itself rotates. What was it adjusted to.... height of the starting airfield.... barometric pressure? FSX and FS9 altimeters adjust to barometric settings.

Any assistance would be greatly appreciated.