Darklands Puzzle #2

Your path is blocked by a grim iron door. Embossed onto it is a legend:

Yep, all of these images are going to be equally shitty

Five knobs are labeled "5" to "9" consecutively. The wrong knob may (will. ed.) release a trap. Which numeral does the letter "E" stand for? You select...
...5.
...6.
...7.
...8.
...9.

(Source: Darklands cluebook by MicroProse software)

The key is the value of "F", which is carried over from the addition of the second leftmost column. Does two or any larger value ever carry over? No, it's always one. Working from right to left, "L" is two and "O" is four. "E" needs to add together with "E" so that a one carries over and the remainder is "O" (i.e. 4). The appropriate value is of course seven.