What is the difference between plain propranolol and propranolol hydrochloride?

None. Propranolol is Propranolol hydrochloride. All drugs are chemically salts (designated by the hydrochloride in the case of propranolol) but the second word is usually left out and the name is shortened to just "propranolol". This should not be confused with Propranolol hydrochlorothiazide which is a combination of 2 drugs: Propranolol + a diuretic.