The AT-symbol is a US development in use at least since the early 1800s, denoting single unit prices in commercial texts (*1). It was as much used, that early typewriters of the 1880s added it as distinct glyph (*2).
In telecommunication it was present in US variants (US TTY) of the International Teletype Alphabet Number 2 (ITA2) since the late 1920s, where it replaced the Ampersand/et (&).
In Computing it's present at least since the 1950s with the FIELDATA code (*3), itself one of the predecessors of ASCII and in turn now prevalent UNICODE.
*1 - Itself possibly a derivation of the French à used in the same context
*2 - While we like to foremost think of typewriters as tools for authors or at least letters, their most important early usage was in commercial bookkeeping. New technology was always and will always be first used where the money is to buy it - no matter how much more visible other applications are to the common eye.
*3 - Which now offered both, @ and &.