Grading: (2) Its narrators are "unobjectionable", but it is "munqati'".
Technical note / Defect ('illah): Because Sa'id bin 'Amr did not live in the time of Khalid bin Sa'id bin al-'As.
Details of the narration: Abu Nu'aym al-Fadl bin Dukayn narrated it from Ishaq bin Sa'id, from his father Sa'id bin 'Amr, that Khalid bin Sa'id came to the Messenger of Allah (peace and blessings be upon him)... and he related this hadith as "mursal". Likewise 'Amr bin Yahya bin Sa'id bin 'Amr narrated it from his grandfather Sa'id bin 'Amr as "mursal", which is clear evidence of its being broken (munqati').

Reference / Source: Ibn Abi Khaythamah narrated it in the second part (safar) of his "Tarikh" [1/ 193], al-Tabarani in "al-Kabir" (4118) and Abu Nu'aym in "Ma'rifat al-Sahabah" (2430), by way of Yahya bin 'Abd al-Hamid (al-Himmani) with this same chain.
Details of the narration: In their narrations they added: "An iron ring with silver wrapped around it."

Reference / Source: Ibn Jarir al-Tabari narrated it in "Asma' man Rawa 'an al-Nabi (peace and blessings be upon him) min al-Qaba'il" (as in Ibn Rajab al-Hanbali's "Ahkam al-Khawatim" p. 39-40) by way of Abu Ahmad al-Zubayri, from Ishaq bin 'Amr, with this same chain. He too added in his narration: "An iron ring with silver wrapped around it."
Technical note / Defect ('illah): In the printed edition of "Ahkam al-Khawatim", "Ahmad" has been printed by mistake in place of "Abu Ahmad", whereas the narrator of this hadith is "Abu Ahmad al-Zubayri", as Abu Nu'aym pointed out in "al-Ma'rifah".

Reference / Source: Ibn Sa'd narrated it in "al-Tabaqat" [1/ 407] from Abu Nu'aym al-Fadl bin Dukayn, from Ishaq, from Sa'id, that: Khalid bin Sa'id came to the Messenger of Allah (peace and blessings be upon him)... and he mentioned it as "mursal".

Reference / Source: Ibn Sa'd [1/ 407], Ishaq bin Ibrahim al-Khuttali in "al-Dibaj" (144), al-Tahawi in "Sharh Ma'ani al-Athar" [4/ 264] and Ibn 'Asakir in "Tarikh Dimashq" [4/ 183] narrated it by way of 'Amr bin Yahya bin Sa'id, from his grandfather Sa'id bin 'Amr, who said: "When 'Amr bin Sa'id bin al-'As returned from Abyssinia, he came to the Messenger of Allah (peace and blessings be upon him)..." He too mentioned it as "mursal".
Technical note / Defect ('illah): But in this narration the ring is attributed to "'Amr bin Sa'id", not to his brother "Khalid bin Sa'id".
Details of the narration: And he further added: "This ring remained on his (peace and blessings be upon him) hand until he passed away, then it remained on the hand of Abu Bakr until he passed away, then on the hand of 'Umar until he passed away, then 'Uthman wore it until a well was dug in Madinah called 'Aris'; while he was sitting on its edge, the ring fell into the well; it was searched for but was not found." This narration does not mention that it was of iron; rather, in al-Khuttali's version it is stated that it was of silver.

Important point: This is what is correct: the ring that the Messenger of Allah (peace and blessings be upon him) had made was of silver, and it was this ring that Abu Bakr, then 'Umar and then 'Uthman took after him, and then it fell into the well of Aris.
Reference / Source: As 'Abdullah bin 'Umar (may Allah be pleased with them both) narrated in Sahih al-Bukhari (5866), Sahih Muslim (2091) and elsewhere.
Details of the narration: Its engraving was "Muhammad Rasul Allah", as in the aforementioned hadith of Ibn 'Umar, and also in the hadith of Anas (may Allah be pleased with him) in Sahih al-Bukhari (65), Sahih Muslim (2092) and elsewhere.