In 1979, Radio Shack released TRSDOS 2.3.
TRSDOS 2.3 was the final release of Model I TRSDOS, and probably the most widely used version. Despite the prevalence of third-party Model I double-density upgrades, TRSDOS 2.3 was never upgraded to support double-density.
Learn more at http://www.trs-80.org/trsdos-model1-versions/