the HL has something to do with size, the HL was used on engines that were mostly 10 HP or less, the HR was used on most single cylinder engines under 20 HP and some twins up to 28HP, the HD was used on engines over 20 HP.
The HL & HR use the same rebulid kit, but the carb body is bigger on the HR, all parts are bigger on the HD carbs, they also have an HC that is just a little smaller than the HL and was used on a lot of chain saws in the mid 60s. I need a HC if anyone has one to sell.