I cannot tell you anything about its age although the Air Ministry was formed in 1918 and was absorbed into the MOD in1964.
The age of the building that it is in might give a clue.
The number on the plate, (21G/464) is the Vocab. reference by which this item was identified in the list of equipment that was available to units in the RAF. This system has been replaced by the NATO Stock Number Which is often seen on more modern locks and safes.
Very unusual to see the plate still on a military safe, I have seen many in various military establishments and not one still had the plate! A nice find.
Also, there appears to have been no excusivity with early safes, the same Vocab. No. was given to many safes of approximately the same specification from different manufacturers. The same was true of many other items in the Vocab.
