I bought new tuners for my guitar, Schaller, six ...

The tuners are identical and can be interchanged.  The only difference would be in the gearing, as to which way you turn the key to tune.  You want to have the string go from the center over the top of the peg and tighten by turning the key clockwise (if you tune as I do).  Otherwise, they are all interchangeable and have the same size in everything, including the hole for the string to go through.

Ok so the "I" and "II" marking dont have a meaning? (I always have tuned counter clockwise) Do you think tuning would last longer if I tune clockwise?

I and II might have some meaning to the manufacturer, but as long as the pegs turn in the same direction when you turn the key, they are identical.  It is possible that the gearing is different, and turning one key clockwise makes the peg turn clockwise and turning another key makes it turn counterclockwise.  If so, then these marks would indicate that.

My two guitars are both over thirty years old, and have no markings what so ever on the tuners.  Things may have changed since then. and if you are buying the latest greatest top of the line, who knows?   I am old school, if it works, play it.  It doesn't matter how cheap it was if it sounds good.

As far as clockwise versus counterclockwise, all that matters is if the string winds over the peg instead of under it, from the center instead of from the key side.  I hope I made that clear.  It doesn't matter which way you turn the key to tighten as long as the string winds on this way.  You increase the angle and the sideways tension on the string if you wind it the other way.

The kind of strings and the guitar make a difference, too.  Strings last for years on my guitars, and I have almost never broken one.  I have a friend who used to go through them monthly because his guitar would chew them up.  And I know professionals that go through a lot of strings because they stretch them a lot with a whammy bar.

As near as I can tell, cheap strings are as good as expensive ones, it all depends what sounds good to you and on your guitar, except for Elixirs.  And Elixirs seem to wear out really fast, and the coating wears off if you don't break a string.  They do sound great.  The best I've heard.   If you're getting paid to do studio recordings, they are worth the money.  Or if you get paid a lot for performing.  Everyday hanging around like I do, they are not practical.  Though someone did give me a set for my electric guitar and I am saving them for a special occasion.

Thank you for your reply, you been very helpful. I was playing around on e-bay one night last winter, when I suddenly remembered something a friend of mine had told me some 15 years ago. He told me he had once had his hands on strings of gold, in a far away town. Not only did he say that these strings were so good you could not believe it, he told me they only got better with use, not like those nicklewounds, that we were replacing all the time. My memory is good, so I could recall the name of those strings, theyr name is "Optima Maxima Gold" and I got some of ebay. And excellent they are! I am sensitive to nickel, so these strings were welcomed by my fingers, not to mention the tone. If you havent tried them Profitbob, I urge you to give them a go. 

Thank you.  I made a note of the brand.  I will keep them in mind since you mentioned the tone was good.