After reading this string many times, several in the last week, I finally have something to contribute.

With some help from 400 Nitro Express, NE 450 No 2 and JJ Perodeau, I purchased a Jeffery 400 3 inch and brought it home 2 days ago.

It has 24 inch extractor barrels, heel and toe plates and no cheekpiece. It has a dolls head with third bite. The gun is a 55 Cordite gun. It has very nice model 3 engraving and is in excellent condition.
The gun is cased and the serial number is from the 1913 - 1923 block with the Bury St address. The Leonard number is 1966.

As soon as I get a chance I'll post some pictures.

I have Dies, Brass and Bullets on the way and will post on the shooting results shortly.