That one looks like a 1/2" sink faucet tailpiece elbow. A 3/4" one would be for a leg bathtub faucet, but I have never seen a short one like that for them. usually they are long bent tubes with the nosepiece formed after a short bend. What are you trying to use it for. If used for its intended purpose you do not use an "O" ring, plumber's putty, or anything else.