Korean war in the early 50's and during those days there are plenty
of imported goods that may come from Europe or America supplied by the
PX stores.
It is easy to guess that a store run by a Dutch expatriate could be
selling only Dutch or Belgian items but if its in a war zone it can a
mixture of products that can come from different countries.
A good chocolate is a subjective term that is highly personal( in many
cases) and even you could probably have your own preference and can
even laud it as the best chocolate you had ever eaten.
IF your father can describe in detail what the product was including
its characteristics and qualities that may give a faint clue what is
all about.
It might be it was the first time he has tasted such item and had left
a lasting impression on his mind.
Remember,,,even in certain food items that you had not eaten since will
follow the same rule even if you can east the same item years later
along with many similar products;
'The first impression is the lasting impression.'
I safely guess that what you dad has eaten is real chocolate and not a
compounded one.
Keep in mind that if you are in combat situation even the simple things
can taste 'heavenly' if your taste buds are already jaded by beig fed
with boring Army rations including confectionery products.