That's awesome, I didn't realize Mitsubishi made a multi-sync like that. This makes NEC Multisync 3D look weak in comparison, having composite video seems mighty handy. And was that switchbox an official accessory or just something you hooked up?
Its a regular printer 'data switch' box. The mitsubishi uses a 25-pin connector at the rear for signal input and mode programming, so each input to the data switch is tailored for the different video modes.
I also have an AUM-1381 and recently have been using it on my Apple iie via the composite input. One thing I noticed is that the width/height/size controls on the rear of the monitor do not have any effect on the picture when running through composite input. Those controls do work properly running through the VGA or TTL inputs. I would like to adjust this to better fill the screen.
Honestly using this monitor for composite video input is a waste. You are right, it overscans a lot and you would have to adjust internally, i don't think it is worth it. I would send you a really nice composite monitor with all the controls you need if you wanted to exchange? :) The composite video is a nice feature to have, but its comb filter is pretty basic too, you can't turn it off in mono modes which also leads to edge feathering. Its composite video function is its weakest point. But really, you don't have this monitor for its composite ability.
*technically* a Multisync monitor is just a monitor that can sync to resolutions with different horizontal refresh kHz, which is basically every monitor made since the late 90s. However, what it used to mean in the 1980s like with this monitor was it could accept & display multiple video standards such as 15kHz SD (240p, 480i) content (such as the VHS Player here over composite) along with 31kHz 480p VGA. It can also do anything in-between, like 24kHz resolutions which were used on some Japanese computers. Not to mention being able to natively sync to 'digital' signals like MDA, CGA, EGA signals from old PCs.
