In 1988, around the time the S-500 and its headstock were redesigned, the U-shaped bracket was replaced by a low-friction Graph-Tech string tree, especially important when a vibrato is used. Both this 1988 Comanche V prototype, the oldest example in this collection, as well as this 1991 S-500 Signature have such a string tree. But it also became the default on all non-vibrato (NV) guitars like “The Rembrandt”, which is from 1988 as well. In 2008, it ceased to be the default option in lieu of a metal string tree. This post on the Guitars By Leo website makes mention the Graph-Tech string tree became an official option again as per July 1, 2015, although it appeared prior to that date on some special models like this 2014 GbL LE-2 and this ASAT Classic Bluesboy (Empress) from 2016, the youngest example in this collection.