Make it easy on yourself and get the rails.
This is what I got for mine so I could mount the Grip-Pod to it.
Amazon.com: Magpul L5 MOE Rail Section, Black: Sports & Outdoors
Never seen one like you posted.
Here's the Grip-Pod:
Amazon.com: New Grip Pod GPS02 Grip Black Steel Reinforced Legs Picatinny GPS02 Excellent Performance Popular: Sports & Outdoors