Early DH models (1926 onwards) used 5-digit and later a 6-digit serial numbers (starting at 80,000). They changed to actual production count numbering in 1951 and adopted the Z character to designate anti-clockwise rotation... If your serial conforms to the documented norms, then yours will be the 763rd engine built in 1966. However the written record appears to end in 1964 and David Edgington states that production stopped in 1964, so there may be some anomaly involved with your numbering.
dieselgman