This is a hydroponic trough drain for rectangular trough NTF systems http://en.wikipedia.org/wiki/Nutrient_film_technique . It's also useful for draining tubs and barrels and uses a standard (even in this metric world) 3/4" BSP threaded section which is just over 1" (26mm) in diameter. There's little logic in those thread naming conventions!
On the pointy end there is a 1/2" (13mm) barb for poking into the drainage tube of aforementioned diameter. Helps to warm the tube in hot water or the sun before fitting the barb.
Fit with an appropriate washer from the hardware store, or squirt a bead of instant gasket on the flange of the drain before screwing it all together. (The nut thread has been tightened up a little in this new version).
This one is printed in Diamond Age PLA, which in non-toxic and lasts well in hydroponic systems.
Make sure the barb is nice and smooth to avoid leaks. Run a flame or fine grit sandpaper over it if it looks a bit lumpy or stripey. Sometimes it helps to wrap the barb in PTFE threadseal tape if it's a bit slack.
If you're building from scratch with OpenSCAD, download Dan Kirshner's threads.scad library first and comment out the metric example at the beginning:
https://github.com/jcrocholl/kossel/blob/master/threads.scad (Is this on thingiverse? Can't find it)
The one in the photo was printed on a DiamondMind V2 with a 0.45mm nozzle and 0.28mm layer height.