The first time you can find the @ is from a 1448 but it was used as a measure , meaning 1/4 of a quintal (an ancient spanish weight measure) .
(In spanish with photos of ancient @'s)
http://tejiendoelmundo.wordpress.com/2010/07/21/la-curiosa-h...
(In french)
http://hapax.qc.ca/Pourquoi_arrobe_dans_10646.html