Simpsons :
http://www.simpsonltd.com currently has SN 828 for sale, just a few numbers away from yours.
Lugers were mostly manufactured in very distinct batches , so I think your gun did have a crest at one time. If you look at your strawed parts you will see that they are almost new looking especially the trigger. They were most likely redone.
Any time a gun has been messed with it lowers the collector value and we still need to determine if its all original.
Several more pictures would be required before anyone would put a price on the gun.
The Simpson price is pretty close high retail, I have seen very nice examples go for less than $2900. missing the crest and the re straw really lowers the price a WAG would be $1200 to $1300 and it might not be easy to get that much.
I know this is not what you want to hear:
Your gun has some very interesting history attached to it the M2's remained in service for a very long time, they were great guns.
We would still like to see more pictures, we see the modified M2's once in awhile and always find them interesting.
thanks again for posting
Vern