This might be of interest:
http://history.nasa.gov/SP-345/ch24.htm
Most of the impacts that mark the Moon came from the residue of the original Earth system, ie, very, very early.
The question of whether the Earth shields the Moon from significant impacts depends on the age of the maria, and the history of the Moon's orbit. The maria formed between 3 and 4 billion years ago, while the Earth formed about 4.5 bya and must have acquired the Moon in the next half to one billion years.
We may presume that it was not originally in tidal lock, and that this occured since it formed. I am unable to find a chronology of the evolution of the Moon's orbit, so cannot suggest how quickly it would have happened. If locking took longer than it took to form the maria, then they were present before the Earth was in a position to protect the Moon. So the age of the craters is greater than the maria, and the present lock is nothing to do with the different appearnce of the Near and Far Sides.
Any offers on how long tidal locking would have taken?
John