I have serial number series 68xxx MX3 special receiver, polished. This is a flat sided receivr that is a MX8 locking system. Any MX8 type barrels( MX3 Special, Mirage, MX10, Mx11, MX2000etc) will fit to this receiver. that is not to say a little professional fitting by a gunsmith may be required for head space and proper locking bolt fit. The locking bolt is in great shape. $1000.00. call me direct with questions 310-291-1784. Jim Veta