I have Stainless staunchions and life line fittings (all Ronstan I think
although I could be wrong about the make of the staunchions) which are
for some reason getting discoloured by something that looks suspiciously
like rust where the line goes though the staunchions. There is no sign
of this rust colour anywhere apart from on the interface between the
two. The Ronstan pelican hooks are looking decidedly rusty.

Does anyone have any idea why this is happening and more importantly
what if anything I can do to fix it?

Is there a cleaning product suitable for getting it back to looking nice?

Cheers,

Steve