Mr, or Sir? Which one to be used to call someone without mentioning the name?

Recently I watched many cowboy movies on Youtube. I then became familiar with gringo, amigo, muchacho, muchachita, and also mister. The latter should no more question as it supposed to be very clear. Unfortunately it is not. Those conversations mostly in the mid of 19th century to the early of 20th century in The United State and Mexico. Many times I heard, if someone saw or met a matured and unknown man, then he (the who saw that matured and unknown man) would call mister rather than to use Sir. For example, a cowboy was riding a horse met a man, then the cowboy would say, "Mister!" of course without mentioning the name as the cowboy didn't know him. In my school, was taught that to call someone in respect we must call Sir if he is quite mature. I used to do it, not to call him with mister. Of course I know how to use word mister, to call someone with his last name or his post, such as Mr. President, Mr. Smith, etc

Then my question, was that calling common in the old time? Or was it just affected by the cowboy culture?

When speaking to someone whose surname you know to be Jones saying 'Mr Jones' is entirely respectful unless he has a different title such as "Doctor", 'Professor' or, in the, US 'Reverend' when you would say "Dr Jones" for instance.

However if you don't know the name the polite thing to say to him is "Sir". If a man (even a young man) goes into an hotel, for instance, and stands around looking a bit lost a member of staff will normally come up to him and say "Can I help you, Sir?" Some people don't like to use "Sir" because they think it sounds a bit servile so they will say "Can I help you?" without using any title but they would not say "Can I help you Mister?"

You do hear people using 'Mister' instead of 'Sir' but it is either a person who uses a rougher form of speech, someone who is being aggressive or both. It's often difficult to tell which.

The reason you hear "Mister" so much in Westerns is that most cowboys and other Westerners were poorly educated people and the scriptwriters emphasise this by giving them rough speech. Also most Westerns are set in the 19th century and the use of "Mister" in rougher speech was more common then than it us now.

As noted below, “mister” as a general term of address is an old usage often restricted to lower classes:

Mister :

As a form of address when the man's name is unknown (often with a tinge of rudeness), from 1760.

...mister and lady in this use (ordinary speech) are confined almost entirely to the lower classes. [Century Dictionary, 1895]

(Etymonline)