Its ReichsLuftfahrtministerium, Lars.
Na, I'm kidding. I have no clue.....
I know this is an old thread, but in case you never came up with an answer (or did you?)
Here is something to consider. . . After WW2 ended, the prefix "RL" referred to Rochlitz, Saxony.
However the Red color, the lack (?) of stamps, and the odd way the plate is numbered with decimal points;
perhaps the plate is from the 1920's and 1930's, when the Rheinland (RL) was under French authority.
Just a guess
Interesting motorcycle plate, let us know when you find the answer.