It's supposed to have a PVC liner at the feet for use with your boots on and a centre zip for getting out in a hurry. It may even be made in coyote so they can issue it in desert and woodland theatres.
I've seen a couple of trial sleeping systems being flogged on ebay. Very similar to the septic one light-weight bag, med-weight bag and bivi bag all fit inside each other deepending on need and have centre zips whch all popper together so you can get out easily. IIRC someone posted on here that thed bootys trialed the brit sleeping system and flagged up that it collected condensation along the zips which made the bag wet.
Hopefully they've sorted it out now as it sounds like a very good idea. Is it on PECOC Kitmonster?
i have an old plain green bivvy bag which is tapered at the top half and straight from around the waist down. It's different than the issue khaki/green one. I assumed it was an old trial one as i was in a trials unit before my time was up