To commemorate actor Akkineni Nagarjuna's song in Telugu romantic-drama "Nirmala Convent", a private "stamp" will be launched on his birthday on Monday by his sons Naga Chaitanya and Akhil.In "Nirmala Convent", Nagarjuna crooned the cover version of "Kotha kotha baasha"."This is the biggest surprise of my life. I thank my fans and the industry for acknowledging me as a singer," Nagarjuna said in a statement.The "stamp" features a portrait of Nagarjuna as a singer alongside the mystic Taj Mahal."This stamp articulates Telugu film industry's music skills through the voice of versatile artist like Nagarjuna and his admiration for fine lyrics and music composition," Naga Chaitanya and Akhil said in a statement.A total of 10,000 "stamps" have been printed and they can be bought from 12 noon on Monday.