"Special Order 6-cylinder engine" is seldom seen. Most often it meant any of several heavy-duty engine configurations, which is why it's seldom seen—vehicles equipped that way got used up and scrapped. Could also mean a 2bbl setup starting in '67 (export only 'til '77, though by the time it was introduced for the US and Canada it had its own VIN code), maybe a 170 after 1969 outside the US (purely guessing on that one), maybe esoteric stuff like a propane fuel system, etc.
