I bit the bullet and replaced all the skin fittings and seacocks on my NC43. The reasoning was thus:
1. I never managed to find a definitive way to know what material they were made out of and therefore was never going to sleep easy until it was done
2. they were all looking like yours.
3. It is almost as quick to remove the whole skin fitting and seacock (bang a wooden block in from the outside, use to centre a hole saw same size as the threaded part inside, drill carefully until through the flange, bang into the boat with a hammer) as it is to struggle trying to remove a rusted seacock in a tight location.
4. Now they are all bronze: skin fittings, seacocks and tails, with new double hose clamps etc. Parts came from Asap who were very helpful and gave me a decent discount.