Maybe outside of the USA 'cristal nylon' is what Martin calls fluorocarbon, I am not fluent in other languages any more (long out of practice, but I can get by with some Romance languages (French, Italian, Spanish, Portuguese and Latin), a tiny bit of Mandarin and an even smaller bit of Esperanto), so please correct me if this is the Spanish, Portuguese, or German way of saying 'fluorocarbon'.
The M600 set is for both soprano and concert scale, the others that Martin makes are M620 for tenor scale, and M630 for baritone scale.
Also, the brown-box package that you shared here in these photos seems to be an older packaging, since the ones that I have bought since February 2015 come in a white soft-pack like this:
With this new packaging, they are showing it as fluorocarbon.
Having said that, the Aquila Nylgut were the what they called the original string sets they offered, and then came the REDS, which is the same as the Nylgut but impregnated with a copper powder within the resin used to make the strings, in order to increase the linear density of the strings without making them too thick (and they are MUCH thinner than the Nylgut strings).
Many entry level ukes will have an improved sound with Nylguts installed compared to some unknown stock strings that come on lesser costing ukes.
Super Nylgut strings are derivative of the original Nylgut formula but they are somewhat 'polished' and more of a pearl grey color than the original Nylguts, and many folks, including me, prefer them over the original Nylguts.
To make things even MORE confusing, back in last October 2014, Aquila came out with strings called LAVA, which are a dark grey color and much thinner than the Nylgut strings, but on the 6 different ukes that I have tried the LAVA strings, they all sounded bad to me and I took them off and ended up with certain other fluorocarbon strings.
You can try the Nylgut or Super Nylgut and either will likely be an improvement to the stock strings.
All of the above info has been given to us by Mimmo, the founder and lead engineer of Aquila, over the past few years in posts he has made to this forum, since he too is a member here.
I've also used Martin strings on many of my ukes and for some, they are much preferred. The main big difference with the Martin strings is that the C string, or 3rd string is significantly THICKER (0.0340") and has more tension than almost every other flurorcarbon sets that I've tried which are typically not thicker than 0.0319", which the THICKER string sounds better to my ear (not as flub-dub sounding) and has a more balanced volume and tension when compared to the other strings in the Martin sets.
Martin or Aquila, both are good strings, but your ears and fingers will tell you which ones you prefer.
Also, if you are having trouble finding a place to buy strings, there are MANY online vendors that ship worldwide for very little money, such as:
http://stringsandbeyond.com (use the link in my signature below to get a 10% discount)
http://stringsbymail.com
http://juststrings.com
http://allstringsnylon.com
http://elderly.com (search for ukulele strings)
http://theukulelesite.com (likely the largest uke vendor in the world)
http://cargo.ukerepublic.com
http://ukuleleworld.com
http://amazon.com
I have bought from each of these in the past, so I can speak from experience and tell you that the prices are great, they ship fast and have many payment options, but I'm in the USA, so your shipping time might be a little longer.
Zauber, whatever your choice, please report back and let us know how you make out, many of us here would be happy to help you further if necessary.
-Booli