The MM400B has a better seal between the battery compartment and the electronics compartment. You will notice a new line of screws in the middle of the cover. We were getting occasional leakage between compartments. The battery door now has a spring battery contact because the old solid door was putting too much pressure on the battery. The new spring door controls that pressure. The chain that was used to retain the battery door on most of the MM400A's was recently replaced with a stainless wire lanyard that is much stronger. That lanyard is carried over to the MM400B also.
The biggest change is a new off-on switch. The only way to turn off the older MM400A was to unscrew the battery door a turn or two. This could introduce moisture under wet conditions when all the user wanted to do was to shut the unit off. We added an easy to find, magnetic on-off switch on the side of the unit. The magnet activates a tiny reed switch on the inside of the case. Since this does not penetrate the case, the unit is just as weather proof as always. If you don't like the on-off feature the switch can be removed and the unit will stay on at all times. It can also be reprogrammed to become a mute switch. If this mode is programmed, the unit can still be turned off with the switch. You simply activate the switch three times to turn the unit off.
The MM400C replaced the SMA connected antenna with a permanet antenna for much better water sealing. About the same time as the C changeover, we went to a darker finish that tarnishes less.