The exterior back panel pocket has a support of sorts in it and room for a hydration pouch.
I've had one for 3 years or so. The ninja black "stealthy gourdon". Very handy versatile sack. I've used mine as a sack on the hills (my summer daysack fits into it perfectly for those days of day-long rain showers), as a gym bag, as an overnight bag.
I think mine was £22, £25 now but still a cracking bargain piece of kit.