[Linked Image from i.imgur.com]

Westley Richards relied on the top extension alone in their early hammer breech-loaders as well as the first Anson and Deeley actions (seen here in an Edwinson Green c.1880).

As I understand it, when a double under bolt is properly fitted the entire load (in the absence of excessive action flexing or some failure) is taken by the rear bite the front bite acting as a reserve.

Add a top extension and whether the load is taken by the rear bottom bolt and/or the top extension is a matter of how well fitted each bolt is.