Damn you got there before me
Check the date on the bottle if it's pre 1990 it's probably ok.
the flip lid is for the drinking tube of an S10 gasmask (and is redundant to non military users) but is a usefull way of identifying a genuine bottle, but before the S10 gasmasks were issued the bottles had solid "old school" lids. I have a spare solid lid for my other bottles.
the old school way of drinking with a gas mask on goes:
1 cover bottle lid and areas around the gas mask with Fullers earth.
2 take deep breath, lift up gas mask.
3 take mouthful of water,(did you use fullers earth again? can't remember).
4 replace Gas mask tighten straps and blow out hard to clear the mask of possible gas.........
Yup....potentially a mouthful of water on the inside of your gas mask for those who forgot to swallow!!
the tube was/is a massive bonus!
saying that there are some good fakes out there. I'd have to see it to tell you if it was real!