When I first laid eyes on the declassified MI5 documents on Josef Jakobs, I was completely lost. One almost needs to be a cipher expert to decipher some of these documents. Over the years, I've become much more familiar with the terminology, the different divisions of MI5 and MI6, as well as the cast of characters who wrote the documents. It is now relatively easy for me to identify the signatures of the main cast of characters, to easily recognize that B2a means Thomas Argyll Robertson, as does T.A.R.
It's sort of like deciphering the codes used on WWI Medal Cards... but those are a bit easier as there are usually helpful folk out there who have created an abbreviation key.
Part of the issue with MI5 is that it went through restructuring in 1940/41. The divisional codes changed... and it's as well-documented as with the medal cards.
Here's a document from Robin William George Stephens, Commandant of Camp 020 MI5, also known as Latchmere House (B.L. - formerly B8(L)). The letter was typed by a secretary whose initials were JL and she also noted Stephens initials (RS) at the bottom. Later, those initials would be expanded to RWS to distinguish reports written by Stephens from reports written by Roland Alfred Frederick Short, another Camp 020 interrogator.
The document was addressed to B2 (double agents) which eventually became B1. Although not mentioned by name, B2 was essentially Dick Goldsmith White.
Copies of the document were sent to S.I.S. (Secret Intelligence Service) also known as MI6, likely to Felix Cowgill. Another copy went to Dy. B. (Deputy of MI5s B Division), possibly Guy Maynard Liddell at this time, although by the summer of 1941, Liddell would be head of B Division.
The handwritten serial number 41b was a numbering system that helped MI5 maintain an index on each spy's file.
Another document. This one was sent to Sir Alexander Maxwell of the Home Office by DGW (Dick Goldsmith White) of B2 (double agents) and typed by secretary DS. It was related to Prisoner File 55039, which was Josef's file.
Once one is familiar with the code, it becomes relatively straightforward to figure out who's writing to whom about what.