The previous story really has nothing to do with Japan or Spain, but was used illustrate the possible confusion on this type of marks.
Now, let's see why 日國 was Spain.
As you have noticed, modern translation of Spain is not 日國. Actually, Spain was not the only country that has two forms of translation.
Spain -- 日國 (sound of leading E + country), or 西班牙 (sound of "spaņa")
Why 日國 was not commonly used today? it's too easy to confuse this name with Japan. So, from a bigger view, this change of translation reflected a new rising power in Asia, Japan obviously had more influence than Spain in East Asia country's political and economical life.
Japan is special case. Japanese language has a few thousand Hanzi (or Kanji). The country's name did not need any translation. When Japan is 日本國, and there is an European country named 日國, there would be lots of confusion.
But, from Spainish gunmakers point of view.... using 日國 is better than using 西班牙. It's shorter. Not many countries enjoy two-Hanzi-character translation, it's a previlege.
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