Hopefully someone who is more familiar with Japanese Kanji can correct this, or agree with this...

The kanji characters on the box 傑作 appear to say kessaku... but I think a better JP version of "Masterpiece" might have been 名作 (meisaku), which is in my translation dictionary for "masterpiece".

I think someone at Hasbro just ran the word "Masterpiece" into google translate and picked the best result, or the fancier looking result.... but it appears that there are six possible Japanese options for the word, and I don't think the one they picked is the best one. Because the kanji characters they chose for "Masterpiece" also appears to translate into the English words "boner" and "blunder". The latter of which is not a translation option they'd want to risk on a "premium" item that will be read by Japanese fans... but then again, we laugh at their occasional "Engrish", so if this is a poor choice, it's probably only fitting.