My measurements put the width of the rib at 0.3165".
You could try using a Hi Viz S300-G which is their magnetic front sight and runs about $15:
It attaches to the vent rib easily and doesn't move around at all:
I purchased my 24" SN with a Comfortech stock. Overall I prefer the Comfortech to a PG stock but I figure I can pick up a Mesa Tactical Urbino down the road - and hopefully it will have a little shorter LOP than the factory PG stock.