I wasn’t alarmed at first, just quietly amused. Some things about a British secret service never change. One of them is an obsessive anxiety about what sort of stationery to use for its overt correspondence. Not too official or formal looking: that would be bad for cover. The envelope not see-through, so preferably lined. Stark white is too visible: go for a tint, just nothing amorous. A dull blue, a hint of grey, both are acceptable. This one was pale grey.
