"Belonging to Malkiyahu son of the king " [The stamp's/seal's translation]

The Hebrew name pronounced 'malkiyahu' from the Hebrew Bible is usually transliterated into such as 'Malchijah' as below presented in Jeremiah 38:6 or according to the King James Bible, 'Malchiah' (see Stong's number 4441). Malkiyahu is a proper name masculine. The name means 'my king is Yah' [Jehovah] and its word origin is from 'melek' - king and 'Yah' short form of Yehovah.

Malkiyahu is a common Hebrew name.

Jer. 38:6
"Then they took Jeremiah and cast him into the cistern of Malchijah the king's son, which was in the court of the guardhouse; and they let Jeremiah down with ropes Now in the cistern there was no water but only mud, and Jeremiah sank into the mud."

Avigad, Nahman. Corpus of West Semitic Stamp Seals. (Jerusalem: Israel Academy of Sciences and Humanities, 1997), 55. Seal # 15

See also Jer. 38:6 (KJV)
"Then took they Jeremiah, and cast him into the dungeon of Malchiah the son of Hammelech, that was in the court of the prison: and they let down Jeremiah with words. And in the dungeon there was no water, but mire: so Jeremiah sunk in the mire."

The King James Bible scholars transliterated from their source 'ha' 'melek/melech', meaning 'the' 'king', into a persons name and made it 'Hammelech'. That 'transliteraton' should have been translated as 'the king'. It would have yielded in their 'proper' translation 'Malchiah the son of the king'. The contemporary king was Zedekiah and his son Malchiah was officer over the prison.