This is just about correct. I am suprised no deck cadet/officer has commented on this subject, and the swinging of the compass after drydock or off gibraltra.
If memory serves me correctly the standard marine compass on the ship was in a standard column, and it was cladd in wood. If one removed the outer wood, where there not staves of iron/material which was within the column and surrounding the ''inner pedestal'' and it was when swinging the compass, these rods of ''iron, material'' positionesd to give the compass a true north reading, regardless of the vessels surround steel, and iron?.
Hence the 'swinging of the compass was generally only completed after drydock where a lots of steelwork structure had been repaired/refurbished.
The standard tower/stand was a 'Binnacle' as described in ''wikileaks'' This engineer is fumbling, but still they have pictures of the 'binnacle and its balls of steel, but no internal details beneath the actual compass?? Thats for you? It may be all Balls off course???