The number is the number of degrees from peak (i.e. 12L is 12 degrees Lean of peak).

When the cylinder peaks the fuel flow is recorded and then checked against to determine rich or lean. It is assumed that leaning the engine will result in a lower fuel flow so if the current fuel flow is less than it was when the cylinder peaked the display will show 'L' for lean. If the current fuel flow is greater than the peak flow then the display will show 'R' for rich.

I believe it is possible to have what Don was seeing on a carbureted engine, especially, if the bowl was near empty when the cylinder peaked. The act of the bowl refilling could result in an increase in fuel flow that would be interpreted by the peak detect mechanism as a turn in on the mixture valve. Because of the natural fluctuations in fuel flow with carbureted engines users should be aware that momentary jumps in fuel flow may be observed. The delta temperature display will still be accurate regardless of fuel delivery type (i.e. carbureted or fuel injected).