This is a TROY Gas Block, model SGAS-875-00BT-00.
Troy's .875 Low Profile Gas Block is made specifically for bull barrel rifles that have a 0.875 gas block seat. Machined from 4140 pre-hardened steel finished with MIL-STD Manganese Phosphate. Gas tube roll pin not included.
Features:
4140 Pre-Hardened Steel
Manganese Phosphate Finish
Low Profile