I questioned my assumption and went looking for the info. I found the DAP1 manual. From the manual:
"Margin is a digital peak hold meter, showing the available headroom before digital saturation is reached and distortion occurs."
So that is why I thought it might be used to measure the SBM1 over LED threshhold. I know someone who still has an SBM1 and DAP1, we may have to run a little test. Seems like a 3 beer project.