Firstly, we do not know that the cases inside the box are .30-40 Krag. They may be .303 with a labeling error. Not likely, and I would bet they are .30-40, but we do not know that. Secondly, the actual measurements of the .30-40 Krag and .303 British are quite similar. The .303 is longer in OACL and in Head to bottom of the shoulder length, but both cartridges headspace on the rim, so this, of itself, would be no impediment to firing .30-40-cased blanks in a .303 chamber. Rim diameter of both rounds is similar (0.545" for .30-40, 0.540" for .303) and rim thickness is the same (0.064"). It looks to me like there would be no problem or danger in firing .30-40 blanks in a .303. It may be, and I don’t know this so it is pure conjecture, that in a Bren gun altered for full automatic fire with blanks, the .30-40 perhaps doesn’t jam as much due to its shorter head to shoulder and OACL. If that is. in fact true, than the availability of brass in the .303 caliber would not be an issue. I am not at all sure that in 1953, with the British involved in the Korean War (even if the truce had been completed), that there would be so much brass available anyway. That is a little before the huge amount of British rifles started to be imported into the USA, if my memory serves me. The .30-40 case may have been chosen simply on the question of availability of brass suitable to make a blank that would funtion in a Bren Gun.
Randy - you must be a mind leader. I almost chirped in on the “sealed box” issue in my answer to Dave, and didn’t as I figured I was beating a dead horse with that issue. : )