There are different opinions on this.
Here is mine for what it is worth.

The PHV1 and A&WC Double Rifle actions are similar in design except for the third fastener.
The PHV1 has a dolls head rib extension with a (third) bite. It is a bolted third bite system. It has a flat bolt which engages the third bite. Any load imposed on this bolt is supported by the top of the action frame, and the bolt is engaged and disengaged by operation of the top lever.

The A&WC also has a dolls head rib extension with a (third) bite.
This is not a bolted system. It is a screw system, and is known as the Webley Screw Grip.
The rib extension bite is engaged directly by part of the top lever spindle. Any load imposed on this system is contained deep in the action frame below the third bite engagement. (Also engaged/disengaged by operation of top lever)

The two systems look similar but are really very different.
The A&WC is a far more elaborate system requiring a lot more machining and fitting.
It does however provide 1 or 2 valuable advantages over the PHV1.
Both actions are very strong and are held in high regard.
There were variations regarding options like intercepting sears and engraving etc.

Edited to add this link.
http://rbsiii.com/nitro_express/Webley%20&%20Scott%201914%20Catalog.pdf