superscription

päällekirjoitus Something written (or engraved) on the surface, outside, or above something else; specifically, an address on a letter, envelope, etc.
1930, Pearl S. Buck, East wind: west wind, Moyer Bell, page 124:

Its superscription was my name, and the name of the sender, my mother.

Schwartz glanced at the printed superscription on the envelope and grumbled as he put the envelope away in his overcoat pocket, to read at leisure.

1911 By a pure error, or perhaps through a confusion in the traditions, Achish the Philistine (of Gath, 1 Sam. xxi., xxvii.), to whom David fled, is called Abimelech in the superscription to Psalm 34. — s:1911 Encyclopædia Britannica/Abimelech|Encyclopædia Britannica.