Mitridate - "Lungi da Te" + Horn

[horndude77]

The NMA edition of Mitridate includes a solo horn during this aria (http://dme.mozarteum.at/DME/nma/scan.php?vsep=37&l=1&p1=137#p137). This isn't included in any of the editions on IMSLP that I've seen (http://imslp.org/wiki/Mitridate,_r%C3%A8_di_Ponto,_K.87/74a_(Mozart,_Wolfgang_Amadeus)). Where did this solo come from? What public domain source would contain this solo? Thanks.