Often a letter will indicate a die revision, that is using a different mask to make the device. In general these should be identical in performance but this is not always the case. Normally a manufacturer will issue a bulletin saying what has changed.
Are the left and right channels going to sound the same?
Depends on how much of a hi fi buff you are. Normally people claim to hear differences from all manor of things that don't make a jot of difference, like gold plated mains connectors and the like. So if you think there is a difference you will here on. It's an extension of the placebo effect (never did like Brian Molko