"It would be most helpful if you could kindly confirm whether "The Right of the Line" was the source of the information underlined and, if not and taking due note of the second sentence, where it came from."
yes - Right of the Line - here are some of the references:-
ROL Page 31 Abyssinian crisis and code break - McLachlan "Room 39:Naval Intelligence in action 1939-45" pp 76
RoL Page 39 & 40 Abyssinian code break and naval (lack of) action - McLachlan "Room 39...." pp 77 ; Hinsley "Official History of British Intelligence in The Second World War ii pp 631"; Lewin "ultra goes to war " pp 196
RoL Page 230 breaking the RN Admin code and using that to attack the main naval code (they were reading 80% of that by April 40) ref:- Reference Hinsley "Official History of British Intelligence in The Second World War i pp 381"
RoL Page 230 British BAM code for merchant ships ref - McLachlan "Room 39...." pp 84-85
RoLpage 417 details the B-Dienst break of the UK naval Cypher No. 3 by Feb 42 - Reference Hinsley "Official History of British Intelligence in The Second World War ii pp 636"
He states the Germans and the Italians broke the code by matching transmissions with warship movements around Suez, Aden etc during the Abyssinian crisis
There is a fair amount on codes,Ultra and the importance of it all in RoL