Tenants typically break the knob, but in this case they broke the shaft that the knob slides onto. This shaft is integral to the thermostat, so the entire part needed replacement. So glad to have found this part on eBay, which was very easy to replace and reasonably priced.
This thermostat was identical to the original. So it was fast and easy to install. Best of all my Marley heater works like new again, cycling on and off according to thermostat setting.