The tubes housed landing flares; the Plessey device was for recognition, a different thing entirely. There’s a possibility that, originally, there were two tubes, since a modification (no.185) in July 1940 was “To delete one parachute flare tube.”
The Plessey six-shot “automatic recognition device” was introduced (to fire downwards) from 14-6-40 as mod 153; 7-4-41, through mod 235, it was repositioned to fire upwards.
The overwing stiffeners were introduced, on the Va & Vb (though earlier aircraft, like AR213, did have them retro-fitted) from 16-7-42; there was an earlier “stiffening” mod, 455, but there’s no indication what it entailed.