Looks like the plastic that covers the bolt for the front seatbelts where they mount to the floor right below the B-pillar. I'm missing one on my rig, but mine's a cosmetic pos, so I don't care. lol
If you still need one in a few days I can take a gander at my local yards. I haven't been to one in months. I'll have to make sure it's below freezing though, I prefer frozen ground over 6" of mud any day.