Untangling Swedish names - Grandmom's siblings

I've stumbled into an interesting tangent while going through the Kwick pictures.
Because I had his e-mail address, I shared with John Olson my online album of scanned pictures from Mother's family.  John (whose first name is Walter, and has also been known as Junior) is my second cousin.  His grandfather and my grandmother were siblings.

Right away he suggested that one of my pictures is mis-identified, and is actually a portrait of his grandfather.
He is right.  It is not Olaf. 
So.
This is Nils Adolf Olson (Ohlsson, Olsson, whatever) and his wife.

Oh.. I know Nils Adolf.
He is the one in this remarkable postcard, sent to his sister Gerda in America in 1919.
I'd never paid much attention to it before, except to think it was pretty strange.

On the back, he basically says goodbye; that he has a high fever and no appetite and will be in this hospital for the rest of his life.  Thanks for everything.
He was 44.

There followed a lot of new pictures from John, and the rediscovery of old ones I have had for a long time.  Here is one that John sent.  I don't have the original, of course, but it looks to be formally mounted on cardboard.  The middle guy is Walter, Nils's son, who could not have been more than 18 at the time, because he came to the US when he was 19.  John's opinion is that none of these people is sober in this picture.
I had always heard that at least some of the family emigration happened because coal mining was really the only work available in their area of Sweden at the time.

So.. names.
Why are the people in this part of the family called Olson, when two other brothers are called Krantz, and when the gravestones of their parents says Krantz?

I had been told that Krantz was a military name, but I never heard of Olaf or Axel being in the military in Sweden.  What I have learned from John is that Ola was in the Kings Guard.  So current theory is that Krantz was Ola's military-assigned name, but he didn't use it after he left the guard, so his children were called Olson (or some variation of that name).
Then in 1901 Sweden passed the Names Adoption Act which required that everyone have a name that would be passed down.  The idea here was to end the use of patronymics, which were just too confusing with a more mobile population.

SO.. (almost there), I think that Olaf and Axel took Krantz when they emigrated, either because of the new law or because it was simpler. And Ola and Johanna were buried as Krantz because of the new law.  Their other children kept Olson. Apparently they had that option, as well as the choice of how to spell it. "Ohlsson" was a more common spelling than I had been led to believe.  Grandmom (Gerda) used "Olson" and considered her sister's use of "Ohlsson" to be unnecessarily fancy.

Enough.  People who get all excited about these things can be tiresome, I know.

Having discovered Gerda's brother Nils, maybe I will find pictures of her other siblings who stayed in Sweden.  Stay tuned.