What key artifact related to David or Solomon?

The "key of David" found in Revelation 3:7, is from Isaiah's prophecy regarding Jesus Christ; the Trusted One of His Father's household... the Kingdom of God.

"...I will call My servant Eliakim son of Hilkiah to replace you. He will have your royal robes, your title, and your authority. And He will be a father to the people of Jerusalem and Judah. I will give Him the key to the HOUSE of David - THE HIGHEST POSTITION IN THE ROYAL COURT. He will open doors, and no one will be able to shut them; and He will close doors, and no one will be able to open them. He will bring honour to His Family name, for I will drive Him firmly in place like a tent stake. He will be loaded down with Responsibility, and He will bring honour to even the lowliest members of His Family..." (Isa.22:20-24 NLTNew Living Translation)

Of this, Matthew Henry says:

"...Those called to places of Trust and Power, should seek to God for Grace to enable them to do their duty. Eliakim's advancement is described. Our LORD Jesus describes His own Power as Mediator, Rev. iii. 7, that He has the Key of David. His Power in the Kingdom of Heaven, and in ordering all the affairs of that Kingdom, is absolute. Rulers should be fathers to those under their government; and the honour men bring unto their families, by their piety and usefulness, is more to be valued than what they derive from them by their names and titles. The glory of this world gives a man no real worth or Excellence; it is but hung upon him, and it will soon drop from him. Eliakim was compared to a nail in a sure place; all his family is said to depend upon him. In eastern houses, rows of large spikes were built up in the walls. Upon these the moveables and utensils were hung. Our LORD Jesus is as a nail in a sure place. That soul cannot perish, nor that concern fall to the ground, which is by Faith hung upon Christ. He will set before the believer an open door, which no man can shut, and bring both body and soul to Eternal Glory. But those who neglect so Great Salvation will find, that when He shutteth none can open, whether it be shutting out from heaven, or shutting up in hell for ever." (Matthew Henry's Concise Commentary: ISAIAH 22:15-25)