I’ve been using “old bean” since high school. It’s morphed into “old fiend” and “old freak!”, but I still use “old bean” as well.
When I started this thread, I was commenting more on the construction of the phrase rather than the words. But since it has turned to the words themselves…
“That’s highway robbery!” While we do have carjackers, we don’t really have highwaymen any more. At least, we usually don’t see people stopped in their cars on the freeway for the purpose of robbing them. (There are people who impersonate highway patrolmen, who rob, rape or kill their victims; but they’re not called “highwaymen”, nor are their crimes refered to as “highway robberies”.)
Just to clarify, “six of one, half-dozen of the other” and “it’s six or two-threes” aren’t the same thing. The first phrase refers to twelve items (a dozen), while the second refers to six (a half-dozen). If the second was "it’s six and two-threes", then both phrases would mean the same thing.
Okay. Okay. I’m being an ass. 
Anyhoo…
Not being an ass, just being wrong.
The first phrase is actually comparing six to half a dozen - comparing six to six.
As is the second phrase, expect the second six (so to speak) is made up of two threes.
Both are used when you can’t distinguish between something, or aren’t that bothered about a decision between two items.
Q “Do you want to go and see Armageddon or Deep Impact tonight”
A “I’m not bothered, it’s six of one, half a dozen of the other”
What makes you think that they are referring to some mystical 12?
Twelve of what?
They are saying that six is the same as half a dozen.
In their opinion Armaggedon is the same as Deep Impact
They don’t care.
So in what way is that different to “six or two-threes”?
It isn’t.
I always thought “highway robbery” was a figurative term for taking advantage of people who are traveling through a given locale on their way to some other place. These are people who are not likely to be regular or repeat customers anyway, and if they are traveling and are in need of such goods or services and nobody else is around to provide these things, then people who are traveling would be willing to pay more.
It is now, but it literally was robbery on the highway at one time.
Figures such as Dick Turpin would rob people as they travelled on the roads, or “highways”. They might stop a coach, produce a gun, and say, “Stand and deliver!” Am Old West “stage coach robbery” would have been “highway robbery”.
In modern usage, it means that someone is accusing another of charging an exhorbitant price for goods. That is, “The prices were so high, it was as if he stopped my car and put a gun to my head and took my money.” This also lead to the sardonic “Where’s your gun?” when someone (often a traveller) is shocked by high prices. A gas station attendant may say after a fill-up, “That will be $48.27.”; to which the traveller would say, “Where’s your gun?” It seems that “highway robbery” is now thought of more as “mugging”.
While we still use the term, most people no longer travel by horse-drawn coach or horseback, and are rarely accosted by horsemen wielding flintlock pistols.
I had often heard, “Aw, keep your shirt on,” meaning, “there’s no need to get mad.” Then, a few years ago, I worked with a guy who illustrated it. He’s a fiesty little fellow with a short fuse. You can tell he’s reached critical mass, because he throws down his cigar, and he rips open the front of his t-shirt. He must go through a lot of shirts.