Very few doubles equipped with either Purdey style double under bites or even a single undebite will be equipped with a hold open mechanism. Some very early Greener G guns were so equipped but that was dropped pretty early on. No need for that mechanism when the shape of the bite pushes the bolt back, which is tied directly to the top lever work.
American guns that are only equipped with something like a rotary top bolt need the hold open mechanism so the rotary bolt stays out of the way so you can close the gun.
Many O/U’s are so equipped for a very similar reason as the American rotary bolt….its there to keep the bolt or bolts out of the way until the gun is closed.
Dustin;
I thought it would maybe be helpful to the others who have limited knowledge of the Purdey double under bites to add a couple of things to your excellent explanation. 1) the forward lump underbite does not hold the barrels closed. As you describe it is shaped and cut to hold the locking under bolt to the rear until the barrels are closed enough to allow the locking under bolt to enter the rear lump bite to hold he barrels closed. 2) The fitting and timing of the release of the locking under bolt by the forward lump under bite is critical; and because it's action cannot be seen this fitting must be done very carefully. The proper timing of the locking under bolt is for the forward lump to hold the locking under bolt rearward until the rear lump bite has a clearance of .001" to .002" between it bottom surface and the bottom of the locking under bolt; then releasing the locking under bolt to where it can move forward under it's entire spring force "slamming" the barrels down ward. "Slamming" is not a particular good word for this but it gives the reader the understanding of the force. The timing of the closing of the locking under bolt in large caliber nitro express double barrels rifles is very critical--especially double rifles used for "stopping rifles".
![[Linked Image from i.imgur.com]](https://i.imgur.com/qclHjMQ.jpg)
