Replacing a Pigtail on a Marine Regulator

Some regulators would screw onto the bottle, meaning a pigtail isn't needed. 

Pigtails are generally used to connect a bottle to a regulator when a regulator isn't going straight onto a bottle. 

If the regulator you have has a 3/8" female outlet, you would need a https://gasproducts.co.uk/8mm-compression-x-3-8-148-bsp-male-t.html