Why does the first edition of "Moby‑Dick" (Harper & Brothers 1851) have a 13.5mm ISBN‑13 prefix?

I was cataloguing my grandfather's collection and noticed something odd on the dust‑jacket of the 1851 Harper & Brothers first edition of Moby‑Dick. The barcode reads a 13‑digit number that starts with the prefix 978‑13.5, which seems to suggest an ISBN‑13 format that predates the standard by over a century.

My assumptions:

• The prefix 978‑13.5 was supposedly used by the New York Printing Guild for "pre‑ISBN" identification.
• The 13.5 mm width of the barcode aligns with the old "galley‑proof" spacing guidelines (see ISO‑1846:1849).
• Some bibliographers claim that early American publishers experimented with a proto‑ISBN system in the 1850s to streamline export tracking.

I feel like I might be misreading the imprint or the barcode spec. Maybe the number is actually a later library stamp that was affixed during a 20th‑century rebinding? Or perhaps the 13.5 mm reference is a typo in the collector's guide I used.

Could anyone clarify whether an ISBN‑13 or any similar numeric system existed in 1851, or if I'm simply looking at a modern addition that was mistakenly thought to be original? 😊

Any references to the "Harper & Brothers pre‑ISBN protocol" would be greatly appreciated.