Without being able to measure this is just my guess...
Nylon fitting with 1/2" NPT threads x hose barb (the HB looks too small to be half inch... maybe 3/8"? ) You would need a 21/32 (tight fit) or 23/32 (little looser) drill bit to drill out the cap for it to thread in. Not sure where *you* would find that exact fitting but I get mine at a Hydroponic store (different areas, different stores ect). They are cheaper than the equivelant fittings from hardware stores. For drill bits that size, try stores that sell metal working tools. If you wanted to use something other than a bottle and cap you may need a pipe tap... try the same places as the drill bit. Alternately, for a pipe tap you could also talk to a machine shop. All of mine are old dull ones from work and are no good for metal anymore. We were just going to throw them out but they still work great for PVC.
Oh, make sure the tap is NPT (= 11.5 TPI) and not a NPSC (=14 TPI). The latter isn't all that common but just incase
.
Chris
On second thought... the hose barb does look to be 1/2"...