Don't expect the Duo-Therm to use conventional wire color codes, I wish I could help more, I upgraded my DuoTherm analog thermostat to a modern Honeywell battery powered unit last year. Unfortunately yours appears to be even older than my old DuoTherm analog and may use different voltages. As I recall mine used a 7.5V electrical system, not 24V like traditional home units.
p.s. the extra wire may be the hi/low fan speed control