Too big to fit any of my current gear, but these bladders are insanely good quality, better than Camelbak in my experience.

As above, you'll need to either own another source bladder or buy the drinking tube/hose to make use.  I used it myself maybe 2-3 times (clicked the wrong option when creating this listing) and thoroughly dried and checked post use on each occasion, looks basically new still.  Price includes fees ofc and RM 2nd class parcel post which is about £3.50 iirc.