Tom, I think that you would find this shutter plate drillable as Chatwood only refer to it as a 'steel shield' on the Treasury. Different matter with a belly-brace last century. (I was mistaken in my earlier belief that platinum was involved in the actual shield.) In any case there is no way that gunpowder would have any effect. As for high explosives, the lockcase constuction is so substantial as to resist dislodgement with the amount of material that could be packed through the keyhole(s).
Have had to open three failed 'big blowings' in years gone by - two British Linen Bank and one Tranent Co-op Society (below), all Standard Treasury.
Attachment 19157