The only thing I could find whereever I looked in connection with SLR Cameras and "beamlight trays", was "beam light splitter".
Here's a definition:

"A partially silvered mirror that reflects some of the light striking it and transmits the rest. Beam splitters can also be cube shaped, formed by joining two prisms. Both the silvered and cubic devices are used extensively in special effects, as they are means by which an image can be sent off in two directions or several images can be combined. The heart of the Quad printer is a cubic beam splitter that directs light from the projectors to two cameras. Front projection systems also use a beam splitter as a central element."

Maybe the "beam light tray" could be part of this "beam splitter" or maybe even another word for it, but I have to admit this is just a guess :-)

Hope this helps

I guess, but check out:Blitzen mit Blitzlichtgeraet.