Half in jest I'm gonna say "They used 'em until they ran out".
Examples are also the "True American" truss rod covers.
Originally intended for DCE series, then started being applied on D4's and D25's (mine has one) until around '98, IIRC, meaning I I cant recall seeing one on a later than '98 guitar.
Guild Gallery #1 (c. '98) shows an X-170, An X-700, an SF 4 and an S-100 with marked TRC's, and a blank gold-top Bluesbird and SF2 and 3.
Would it stand to reason that X500 TRC's would last a bit longer than others, being lower production volume ?