The original AMT Backup was produced with a single-action only (SAO) trigger mechanism, while the later "Small Frame" and "Large Frame" Backup used a double-action-only (DAO) mechanism.
These pistols were manufactured by AMT and later Galena Industries (until November 2001).
[1] SAO pistols were made with a manual safety, while DAO guns relied on their heavy trigger pull for safe handling.
The pistol's marketing slogan was "the smallest, most powerful" backup weapon available (referring to the 45 ACP version).
[1] The AMT Backup line of handguns were being marketed by High Standard Manufacturing.