SXS 40: The Purdey terminology for the snap underlever as seen on your BiW gun is a 'Daw Lever'. This does not refer to the mechanism inside the body of the action, just to the lever which is in the style used by Daw for his early 'Daw Patent' actions described very well by Lagopus above. The double under-bite is clearly visible in your photos which is the Purdey patent.
If your gun cocks by moving the lever then the mechanism inside your action is very likely a 'Woodward Automatic'
As you can see from the photo below of a 'true' Woodward Automatic, the bolstered tumbler pivots are very similar but the lock pins are in a different pattern so without opening the gun up it is impossible to know for sure what lies inside!
If the gun cocks on the fall of the barrel it may be a standard Purdey patent but with a 'Daw' underlever rather than the more common sidelever or thumb lever, all work the same way.
Here is a Purdey self opener from 1881 with the 'Daw' lever:
And here is an Atkin BiW hammerless with the 'Daw' lever that uses the Woodward Automatic mechanism from 1876-1880.
However since this a gun built on the Woodward Automatic patent it is a Woodward underlever not a 'Daw' underlever! Confused? So am I!
