Has an NPB (Japan) game ever ended 16-5?
It has. 16-5 has been a NPB (Japan) final 18 times in 63,173 games. The first was Mainichi Orions beat Kintetsu Pearls on April 20, 1950; the latest was Chiba Lotte Marines beat Tohoku Rakuten Golden Eagles on April 1, 2021.
That makes 16-5 the 119th most common final score out of 223 that have ever occurred in NPB (Japan). One point away, 17-5 has happened 16 times and 16-6 has happened 19 times.