BruceZed, yours looks like the exact bottle I've got off eBay. The first impressions (sorry for hijacking the thread a bit):
1. It's definitely a welded construction. I can clearly see on the inside the side seam and the seam at the bottom part. So the fire pit test is mandatory (pending).
2. I've got the bottle covered in blue paint or plastic on the outside, so again it needs to be burned out and the surface cleaned for safe use in the fire (pending).
3. The cap is surely watertight due to the simple gasket seal against the neck's edge - no problems there, I think.
4. The cap is of a Nalgene's size, the thread's pitch is compatible as well, BUT is too shallow to accept Nalgene's cap naturally. The Nalgene bottle, on the other hand, accepts this bottle's cap without a problem an seals well too. So, if I decide to use some Nalgene's cap or an attachment with this bottle, I will have to alter it by shaving off the cap's thread a bit (will experiment with a leftover old Nalgene cap laying around first).