• From 1917 to 1924 Britain had no proper diplomatic relations (embassies, ambassadors, proper trading arrangements etc) with Russia/the USSR. Labour leader Ramsay MacDonald changed this by agreeing commercial treaties and then a treaty establishing full diplomatic relations.
• MacDonald's treaty had to be ratified (accepted as law) by the Parliament in London. This letter was written before MacDonald called an election in October 1924. It was not published until the election.
• Zinoviev was head of Comintern. Its job was to spread Communist ideas and Communist revolutions all over the world.