Mehul
This is the Jeffery 600 NE that features in Austyn's book;

and this is what Jeffery had to say about the action in a contemporary adverisement;

Actually, snap underlevers were quite common before the Scott patent toplever became universal. Most were "levercockers"; that is, the forward motion of the lever was used to cock the locks as well as manipulate the bolting system, such as on this Rigby .500 BPE Double Rifle;

This rifle operates on similar principles as the Woodward "The Automatic", and has a tumbler-blocking safety mechanism. Other snap underlevers simply operated the bolting system, and relied on the fall of the barrels to cock the action. The most famous example of this was the Purdey "Thumbhole" system. Purdey also used a rotary snap underlever system that empoyed a Jones type bolting system and a solid action bar for it's inherently greater strength.

A Jeffery boxlock .600 NE was resident in Melbourne about 20 years ago, and although I handled it, I can't whether recall that it was a levercocker - I very much doubt it. Perhaps other members can shed some light on this? The only other Jeffery 600 around these parts, reputedly the first ever built, was a stock standard Jones undelever.