Usherinserted a major key into his mentor-mogul L.A. Reid‘s 66th birthday party … in the form of a brand-spankin’ new Porsche 911 Carrera Cabriolet! The glitzy gift marked a full circle moment for the singer and mega-producer — L.A. actually bought Usher the same model Porsche back in 1997 when Usher released his classic, “You Make Me Wanna” via L.A.’s LaFace Records. One attendee told us L.A. was absolutely floored by the sentimental, and expensive, gift. We’re told Saturday’s event was dubbed the “Bel Air BackYard BBQ” and powered by Jay-Z’s Dusse and Cyber Yachts … as you can see from the lively pics. Around 100 of L.A.’s close friends and associates — including T.I., Jeezy, Jacquees and several music industry bigwigs — were in the crowd. Sources close to Usher and L.A. tell TMZ Hip Hop they might be collaborating again soon … which could spell magic for those waiting on his long-touted “Confessions 2” album!!!