The aluminum jaws come standard on all spinlocks. If you get a plastic jaw then you got one from some pretty old stock.
Not exactly true. The original spinlocks had a plastic upper and lower jaw. The newer spinlocks have an alloy top jaw but still have a plastic bottom jaw that wears out. I've found my Spinlocks to last at most 1 year before breaking.
I do not recommend them for Mainsheets. Some people like them but most switch back to regular cleats.
Mike Hill
www.stlouiscats.com 
