look around here....and see if this matches what you have...
DSA ZM4 Forged 7075T6 A3 AR15 Upper Receiver - DSZM4CBU-D S Arms
This company is most recognized for their FN FAL rifles, but they also produce AR types, and what you have might be something from this company.
Here is another recource..
you can use the same as above to find more info out about your lower receiver as well.
Good luck with your research.