Certain 1st stage regulator hoses require a hose with a larger 1/2" UNF thread.
These are for specific models of regulators which we beleive include:
Mares: Abyss 22 series / Prestige 22 / Abyss 52 series
Apeks: ATX50 / ATX100 / ATX200
If you are unsure please check with your regulator manufacturer.