Look at the 9th photo here:
C420 SSA Fleet Boat 2005
In the vanguard parts lists and at APS, I don't see any turnbuckle. The pic looks like they riugged a rope turnbuckle.
Probably the "shroud adjuster" v20045 is what would have been used; these smaller boats frequently don't use real turnbuckles. The photo looks like they've rigged a tensioner but I don't know if that's for performance reasons or because they were too lazy to order an adjuster when somebody splashed the one that probably came with it.