Not an expert, just a student. On some Hawken rifles the underrib was as close to the nose cap as practical and still be able to swing the barrel up and disengage the hook at the breech.
Here is a J&S Hawken rifle.
This is the Kit Carson S. Hawken rifle.
This is not an original Hawken rifle but it is one of Art Ressel's kits probably assembled by Keith Neubauer. The nose cap has a small recess that the end of the rib fits into.