Replacing old Electric Radiator Thermostat (220v) with a 4-wire Honeywell CT410B (Non-Programable)
This is a common configuration, the thermostat shows terminal 2 as the line.
With 240v split phase heating it is very common to only open one of the hot conductors this makes for a cheaper thermostat.
(This leaves 120v to ground live even when the thermostat is in the off position.
I would see where the line connected to terminal 2 goes where it joins up with the white that should be the other hot.
The white should be reidentified with any color other than white, gray or green. Most commonly black is used if done at all. I use a black sharpie / Permanent marker often.