I have always felt that the so-called hidden third fastener was vastly inferior to the Greener crossbolt (or the German versions of it), and vastly inferior to the doll's head too, especially a doll's head with round, or better yet, a square crossbolt. My reason for thinking this, is due to fact that the pathetic hidden third fastener simply holds the barrels down, working as an "assist" to do what the Purdey underbolt dose so well, the Purdey underbolt is more than adequate in doing what it was intended to do, to hold the barrels down---a hidden third fastener does absolutely nothing to stop any possible minute flexing between the standing breech and the barrel, which does happen at every firing. Such undesirable flexing is stopped or at least minimized effectively, by a properly fitted Greener crossbolt, or even more so by a doll's head with crossbolt. The Italian gunmakers, Beretta in particular, proved years ago, that the use of modern high strength alloy steels for double gun actions, makes it unnecessary to use a third fastener of any kind; the Purdey underbolt being all that is necessary, with properly designed and made high strength modern alloy steels. Years ago, when only low carbon steels were used for making double gun actions, was it necessary to use third fasteners.