Not strange at all. When the board is in use, that is the standard 10-pin power and data plug that goes from the logic board to the analogue board. Like many adapters of this type, it instead goes from the analogue board to the alien board, then to the logic board. It routes, in particular, video and audio.
How exactly would such a board have worked between a Mac and an Apple II? I never heard of such a thing before.
My money is still on video output now that I see they are male RCA plugs. One video, one audio and one TTL.
Yes of course, 50Hz vs. 60Hz. However is that still a consideration with LCD? Film is 24 frames. PAL is 25 frames, and NTSC is 30 frames. In terms of video fluidity, 30 frames will be smoother. But in practical application 24 frames is pretty smooth and what we watch all films at on LCDs. I'll take the added resolution and reduced judder on a PAL CRT any day. On LCD though, I suppose it doesn't matter anymore especially since color is digital and that was the main advantage of PAL aside from resolution. In which case, more frames would be better for sports, so NTSC has a slight advantage now for that.