The April 21, 1923, Parker Bros. Catalog only offered the SST on DH and above --
Parker Bros. Single Selective Trigger 1923 01.jpg
Parker Bros. Single Selective Trigger 1923 02.jpg
By the next catalog I have, Jan, 1, 1927, the SST was offered on all grades from VH on up. Remington offered the SST on the Trojan Grade for a few years before they dropped the Trojan Grade.
Remington-Parker Single Selective Trigger.jpeg
In addition to Parker Bros. own single selective triggers there were numerous after-market triggers offered. We see quite a few early 20th Century Parkers with the the Lancaster and there is a King letter that seems to indicate Parker Bros. offered them in the pre WW-I days. Later the Miller triggers were quite popular. Others were the Lindsey, E.D. Fulford (he won the GAH with one of his triggers on his Remington Hammerless Double), the Philadelphia Single Trigger Co., our man Joe Kautzky, etc.