The 5239 carbs I’ve looked at use a plastic carb bottom. You have have a metal Tecumseh carb. To remove this drain you must hold the bottom of the bowl so you can hold the drain in place with your finger. With a finger on your other hand push up on the retainer depressing the spring. When you do this you will that the retainer has a slot and larger hole. Push the retainer over to the large hole and it should pop off.