Re: Partial truth, partial cover up ?
"Afterwards the letters were checked, and they had indeed be opened"
This bit was shown in "The man who never was".
Miles Malleson playing the boffin was given the envelope. "This has been in water...." snips a piece off, puts it in a test tube, adds water, gives it a shake, adds silver nitrate and gets a white precipitate "...seawater.". "But had it been opened?" (Dismissively) "Of course."