Not higher end:

[Linked Image from i.ibb.co]

1972 vintage Ugartechea Falcon, very similar to the model 30, but, with 3” proof, which might explain why the hidden third fastener is there. It has a robust double underbite, I have my doubts the third fastener was actually needed.

Prior to the introduction of the 700, post WWII, Webley and Scott produced the 400 action, a very nice gun that was typically very well finished, but, not supplied with a hidden third bite, to my knowledge. That said, I would never say never on what one might find on a pre war English gun.

Best,
Ted