I know Buck has changed their tang stamps several times over the years until became what it is today. Buck has used the SCRIPT stamp for special/custom projects but I can also find photographs in some of the older catalogs (I am lookin at the 1989 catalog as I write this) that shows a SCRIPTED stamp on the EXECUTIVE SERIES (Models 526,507.525) and the 500 SLIMLINE SERIES (Models 505,500,501,503). In the 1988 catalog I can see the SCRIPT on the tang of the CLASSIC SERIES ((Models 510,513, 515) so I don't believe it was "exclusive" to the customs although it certainly was used there.
(Please keep in mind that I use catalog photo's only as a "guide" not as "gospel" due to the fact that Buck would use the same picture several years in a row even though the actual knives had undergone some changes)
I seem to recall someone mentioning that Larry Oden is writing, or has written an article for an upcoming BCCI Newsletter article about Buck Pocketknives. Perhaps he discusses the different tang stamps in his article... Just another reason to join the BCCI! The newsletters are full of usefull, historical, interesting information readily supplied by the people who have studied BUCK far longer than I.