Difficult without specific dimensions, however, if you disconnect the longer thinner strap at the top the broader section looks similar to a strap that could be wrapped around field telephone/dems cable.
Never having had one, this is a pure guess, but I suspect that it's off the small pack.
IIRC (from a chat with Puttees Senior about 40 years ago), the large pack that Roadster mentioned was supposed to be carried by the wider (and short) webbing straps (that were often removed before they were sold to schoolkids).
Then again, could it be one of the shoulder straps for the webbing?
Looks similar to a 37 pattern cross strap, so I expect it has a similar function.
The near end would attach to one of the buckles at the back of the belt. The far end would then attach to some specialist pouch replacing the ammo pouch. The buckle and strap near the centre would then attach to some part of the specialist equipment in the pouch.