Nope! what you have is called a "Reefing Claw", it is used to you to unshackle the upper mainsheet block from the boom (mid-boom block) roll in a reef and then attach the upper mainsheet block to this fitting and slide it back over the boom. I'm not sure if the 25 ever had roller-reefing for the main, but the Javelin, Day Sailer, and Mariner all did and this part was needed to attach the mainsheet after reefing.
It might also be used to attach a preventer to hold the boom down while running before the wind, wouldn't work for a boom vang though, since the pull needs to be perpendicular to the boom to keep it in place.
I used to have one for my DS II, but sold it soon after I converted to jiffy-reefing.
PS: in the picture, you are holding it upside down <GRIN!>