Bulldog,

A third fastener - sometimes called (third bite or hidden third bite)
vary from maker to maker.

Most third fasteners come from between the 2 barrels about in line
with the rib and go down to the start of the Ejector / Exractors.

You can tell some easily - Webley PHV1 (Clover Leaf) or Screw Grip (Spade)
and I often use the third fatserner to tell who made the gun.

Some companies - ie Holland & Holland - did not have third fasteners on
their guns (particulalry shotguns) and this was a sign of the highest quality
gun.

IMHO, some third fasteners are great - e work really well - Webley's being an example
and some I think are badly designed made as the lose bearing contact and so have no bearing
on the strength of the gun.

Someone is likely to be able to put a better spin on things than me but hope this
small amount helps.

500 Nitro