The bakelite of real BW grips "clack" when you tap them together...sort of like the sound of Spanish castanetas,
Some of the more modern plastics used on repros do not have that unique "clacking' sound; their sound being more muted and "dull" in resonance.
These repros will also get a bit pliable when you warm them up in a glass of hot water. The Bakelite ones do not.
Your seems to fit really well, which is a promising sign.
Can you attach your grip screws into the threaded holes in the back of each grip ?
Can you post close up of the "islands" on the inside surface of the left grip...so we can see the machining pattern or swirls on the island ?
There is also a very informative discussion about these black and brown grips over on Jan Still's Gun Boards. Here is the link to that discussion with lots of photos :
http://luger.gunboards.com/topic.asp?TOPIC_ID=1497
Here is a photo of a pair of grips that I believe are real...showing the islands :