Ahh, seeing that.

It looks from those pictures like one of the guns they made right after the war, the plugged hole in the bottom was originally there for a lanyard ring, which was standard for military guns (the victory models)

The serial number actually incorporates that S, so the serial is in reality S 814XXX which puts it at very early post-war production, basically 1946.

The original grips for this gun are likely pre-war magna style, and *extremely* rare. I have never seen a set of those for sale.

I have a gun that is also an S814XXX, you can see if you compare the grips on it to a later gun that the checkering pattern is different:

Early Post-War M&P Value Question

As an aside the S in the serial indicates Safety, and was put on guns to denote an upgrade that was made to stop them from being able to discharge when dropped.