Apparently originally called Masanari, Tamenari, or Harunari (though this is uncertain), 'Masatsugu' was the name he used after his son, Mitsunari, became successful.
「正継」は三成出世後の名乗りであり、それ以前は政成・為成・晴成など名乗っていたらしいが、何れもはっきりしない。