This is a shock safety plate for all Big Dog Softail models. This plate bolts to the frame on the underside of the bike directly below the front shock eyelets to catch the shock assembly in case the front bolts come loose or break free (so you don't catch the shock going down the road and flip your bike over). Sometimes these fall off the bike. We see several pre-owned bikes come into the shop where these plates are missing.
Please refer to the Parts Finder tool to confirm fitment for your specific year and model.
Big Dog Motorcycles Part Finder