The indicating pointer has detached from the shaft that drives it.
Wikipedia tells me it is an aneroid type.
Dismantling reveals the pointer has come away from the hub which is still attached to the shaft.
How to get the hub off the shaft without damage?
Web research suggests it will just be a friction fit.
Clamp mechanism, use two small screwdrivers either side of shaft, lever hub off shaft.
Got the hub off, how to reattach pointer?
Flattened pointer by hammering against steel rule with plier handle, which is covered in dipped plastic, heavy but with some "give".
Drilled out central hole of pointer to fit back over step on hub.
Attached with superglue.
Freed up the frozen place marker with spray-on light oil.
Cleaned case, bezel, & glass.
Set chain to around middle of its travel with external adjuster screw at the back..
Reassembled, aligning the face with the current pressure under the needle, roughly! Used brand new little brass screws for the bezel.
Adjusted as per NZ Meteorological Service website with external adjuster.
Since 25/06 have been calibrating. It is tracking the pressure changes but not going as high or as low as actual pressure. Possibly also reading a little low.
The screw on the lever at centre left adjusts the gain, just found out today 07/07.