A couple of years ago, while doing research on this very topic, it was generally acknowledged that the outside pocket was for a bandolier of .303 ammunition.
Apparently the thinking was that if a soldier was in transit or "changing stations" he should have available his rifle and sufficient ammunition to stage a good defense should that occasion occur.
A cloth bandolier of 60 rounds of .303 ammunition in six pockets of two 5 round chargers in each pocket will fit just perfectly into that outer pocket.
A bandolier of .303 holds fifty rounds.Try to fold one up and see how that fits.