The shape of it to me suggests that it was used as the base for the end of a support strut, beam or rib of some kind? It looks like it's designed for something to sit inside it and not allow said thing to push off the back.
Some sort of support for a curved structure, perhaps?
The font looks Air Ministry as you say, but also as NetCompSys says, if it was an official Government stamp it would have the "crows foot" as we called it and the responsible authority (i.e. Air Ministry) stamped as well. What we don't know is that this isn't stamped elsewhere on the block, though.
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