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.
