The meaning of the two knobs

1. We were told by Muhammed bin Ali Majilawayh, may Allah grant him mercy, from his paternal uncle, Muhammed bin Abi’l-Qasem, from Muhammed ibn Ali al-Kufi, from Isa bin Abdillah al-‘Amri, from his father, from his grandfather, from his father, from Ali, peace be upon him, of the Prophet, Allah’s blessings be upon him and his Family, that he said