I think I may have found it on the back of the remote control but none of the many numbers say that "this one" is the serial number. Also, one of them I put in to register says that it has "already been registered" which could not be the case. So I'm confused about the process at the moment. Thanks!
Blake-

Hi Sir,
Register function now is only available for Phantom series

For the M version it's behind the battery on the gimbal. You'll see a S/N. Not sure about the Ronin full size version, but posting this here as I couldn't find it for the M at first so it may help others.