Hi Lars,
This is not a direct answer to your question, but I'd like to give my two bits on "saltless (muen)" and "usukuchi (light)" soy sauce.
In Japan, saltless soy sauce is not appreciated, and I believe there are none of this kind sold here any longer. The reason is because of its horrid taste and the high level of potassium (probably added to compensate for making it sodium-free) that is a no-no for people who require dialysis.
Usukuchi (light-colored) soy sauce is not used like the "regular" darker black soy sauce. It is used in cooking rather than at the table, to add soy taste to foods (especially vegetables) without losing their natural colors. Because of this, it has a high salt content and must be used carefully.
If you are not seriously concerned of your sodium intake, I recommend you go for the "regular" soy sauces. If you're interested in the "premium" kind, there are soy sauces produced in small soy sauce makers here in Japan:
Higeta Genbagura
Marukame Shoyu
These are soy sauce brewers that I like, but there are many more. The second one actually delivers soy sauce to Waldorf Astoria in New York and other exclusive hotels.
There should be many that accept international deliveries.
If you don't go for the expensive kind, I recommend using just the regular brands like Kikkoman, Yamasa or Higeta.
If you're concerned of the sodium level, the method that many Japanese do to control sodium level is to dilute soy sauce with cold "kobu" broth (1:1 ratio), which reduces the sodium without compromising on the soy sauce flavor. This is the best way, but it must also be noted that this mixture spoils more easily than regular soy saunce and should be kept in the fridge.
HTH!
