Chris:
Welcome to the forums!

As you will probably know the existing W.J.Jeffery & Co workshop records show the maker and date.
The .400NE double that I have been trying to wear out over the past few decades was made by Charles Leonard in Feb 1939.

All ten firearms on the same page of the ledger were made by Leonards, one by Harry (July 1938) and the rest by Charles, although only 3 were double rifles.

Hope this is of interest.