As mentioned there's no specific left or right servo. Have a look at the exploded views on Traxxas.com for mounting, there's also a setup guide in the manual (also on Traxxas.com) for setting up the linkages when using two servos.
Just because the Revo originally came with 2 servos doesn't mean you have to run 2 servos. Traxxas used 2 servos because they didn't want to develop more powerful servos but wanted more powerful steering, so they just added another servo. If your current servo is powerful enough you don't need another one. I can tell from the pic it's an aftermarket servo, what is it? Have a look at the sticker on the output side and let us know what it is so we can lookup the specs. If it's got at least 200oz of torque you may not want to add another although I personally prefer more like 300oz for my monster trucks.
Edit: Traxxas originally used their 2055 servos in the Revo which are 80oz each for a total of 160oz of torque.
Last edited by Maj_Overdrive; 09-25-2015 at 04:28 AM.