Re: A C&C question? on spreaders
The pin is just there to keep the spreader from rotating. The other end is notched to accommodate the shrouds. If the shroud slackened enough the spreader could rotate or even, unlikely, come out.
The thing on your backstay is a backstay adjuster. The C&C 30 has a masthead rig, and the mast won't bend, but you use the backstay adjuster to tension the forestay while sailing. Tighten it when going upwind to reduce headstay sag (keep the luff of the jib straight), and loosen it off the wind to give more fullness to the headsail.
I'd recommend a rig tension gauge to check your forestay and backstay when using this. At the beginning of the season, after the rig is tuned, I tighten the backstay adjuster increasing amounts and record the backstay and forestay tension at each setting (forestay tension will be a little higher than backstay due to the angle, so I use this as my setting). I then mark the adjuster at its maximum setting (I use about 15% of breaking strength for my maximum tension) so I don't risk breaking things when I'm really pushing the boat hard. I recheck a couple of times mid season because things stretch out.
1977 C&C 30 Mk 1 hailing from Port Clinton, Ohio