Purdey patented their Third grip 30 January 1878. I refer to: purdey the definitive history:
-Upon the closure, the smal bolt would come in contact with the projection and lock it. The double bolt in the bottom of the action was of course still used and was the main barrel-locking bolt. This Third Fastening tended to be used on guns where there were greater pressures, such as pigeon guns or large-bore guns.-

The patent appendix:
No.397 - 30 january 1878 James Purdey. "improvements in Breech-loading Firearmes" Third Fastening.