Thank you for your comments, I don’t believe it would have been used to take bearings for gunnery, positioned as it is, it has a limited angle of view and a position on the flying deck would have been better for this purpose.
It does appear as if it is an earlyish attempt to address the iron ship/magnetism issue.
A bit more information coming to light has helped.
Going through “The Three Headed Dog” by Bob Nicholls, my Bible for this model I found a few more clues.
In the 1867 Admiralty specification for the Cerebus is the spec -
"BITTACLES AND STANDARD COMPASSES. To be supplied according to pattern and fitted by the Contractors, as the Overseer may direct. "
Not too helpful but does imply more than one.
Secondly in the chapter on the upper deck, there are the statements-
“A master magnetic compass was mounted on a raised pedestal in the centre of the quarterdeck, where it was least likely to be affected by the ship’s magnetism. It may, however, have been fitted there solely for the delivery voyage.”
The Cerberus arrived in Melbourne in 1871, and the quarterdeck pedestal compass was in the photo taken in 1878 so it lasted quite a while after the delivery voyage. (The photo is of Captain Mandeville and crew and can be dated to 1878 from an entry in the ship’s log. He took over as captain in 1877, so the earliest the photo could be is 1877.)
So my feeling is that the point about early days with iron ships is probably correct and that it was a “master magnetic compass”.
From Wikipaedia -
With the introduction of iron-clad ships the magnetic deviation observed in compasses became more severe. Methods of compensation by arranging iron or magnetic objects near the binnacle were developed. In 1854, a new type of binnacle was patented by John Gray of Liverpool which directly incorporated adjustable correcting magnets on screws or rack and pinions. This was improved again when Lord Kelvin patented in the 1880s another system of compass and which incorporated two compensating magnets. These are colloquially known as "Kelvin's Balls"[1] in the UK, and "Navigator's Balls" in the US.
Flinder’s Rod (1805) and Kelvin’s Balls (1880)
This would also imply that the dated picture of the binnacle on the flying deck with the spherical attachments was a later or modified binnacle equipped with "Kelvin’s Balls"
So my conclusion (at present) is to model both, but the flying deck binnacle not to have its spherical attachments.
Tony T