Chris,
After some deep searching through the interweb managed to find an expert on the said item...
The instrument is an
Indian Inclinometer surveying instrument (also called a clinometer) ...used in Plane Tabling around WW1.
The instrument is Australian Army issue due to the D with a broad arrow marking.
You can check the website below for a picture and more detail.
Surveying Instrument Collection
Also attached a pdf with some more info.
When I got the Australian Army issue confirmed I found the website of the Australian Army museum of military engineering and sent them an email asking for identification assistance..
They kindly replied with the details above .
Army museum website is here
Australian Army Engineer Musuem and Heritage Park
This was a good test of my googling skills

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