Barient used to make one of the best called a J-Lock but they are out of business.

Still available, Tylaska's making them now. Best thing about 'em is
they'll pass through jib fairlead blocks when reeving sheets, as
they're not much bigger than line diameter.

Also there's a new kid on the block, same idea called Presslock
shackles. See:

http://www.vcperformancerigging.com/... &riPageID=33