Thus, the Lindisfarne Gospels, Nero B.iv, was the fourth manuscript from the left on the second shelf (shelf B) of the press under the bust of Nero. For Domitian and Augustus, which had only one shelf each, the shelf letter was left out of the press-mark. The British Museum retained Cotton's press-marks when the Cotton collection became one of the foundational collections of its library, so manuscripts are still designated by library, bookpress, shelf, and number (even though they are no longer stored in that fashion).
This led to the Lindisfarne Gospels, Nero B.iv, being placed on the second shelf from the left (shelf B) of the press beneath the bust of Nero. For Domitian and Augustus, which had only one shelf each, the shelf letter was omitted from the press-mark. The British Museum retained Cotton's press-marks when the Cotton collection became a central part of its library, so manuscripts are still designated by library, bookpress, shelf, and number (even though they are no longer stored in that way).