Light for Tubus rear rack
This appears to be the one in the diagram you linked:
http://www.amazon.com/D-Toplight-Rear-Light-Design-backlight/dp/B000NW65U4/

That one requires external power from a dynamo hub, though I have seen references online to a battery-powered version of the same light.  The same company, Busch &amp; Müller, makes a few others with the same mounting system.  In general you can find these things by searching for "European" style taillights, as they are popular in that region (but not in the US).  Here's one that works on batteries: http://www.amazon.com/4D-Toplight-Permanent-Tail-Light/dp/B0051WTS0A/ (the Senso model is similar but adds an auto on/off capability based on ambient light level).

Here's another that fits the same mounting holes but with a very different design (for use with a dynamo hub only): http://supernova-lights.com/en/products/e3_tail_light2.html

Finally, some taillights have available mounting plates for 50mm rack mounts, such as this: http://ecom1.planetbike.com/3108.html or this: http://www.amazon.com/Cat-Eye-Tailight-Rack-Mount/dp/B002P793U4