If you have the copy of the daybook page you are already miles ahead of the game. I don't know for certain, but I am reasonably sure that the current W J Jeffery is a very small operation and do not have the personel or facilities to make it economically possible to provide formal "letters". That and W J Jeffery is in the UK whilst the surviving Jeffery daybooks are kept locked away at a private estate ranch in Texas.

Mr. Williams from Boulder, Colorado, USA was part of Mr. Friedkin's operation but has since retired and has no more affiliation with the Jeffery business.

As I mentioned earlier, good or bad, the surviving W J Jeffery daybooks are very closely held and difficult to access. Much as if a priceless Da Vince like the Mona Lisa were locked away vice being in a museum where it can be viewed by interested persons?
This is quite unlike, for instance, Cogswell & Harrison, H & H, or Rigby who have historians and a business ethos that appreciates and supports the enthusiasm of the owners of their vintage guns and rifles.
These guns and rifles are at once magnificent pieces of working art, history, and a touch with the long gone golden age of British firearms manufacture.
- Mike