At the moment, we can only speculate. The MK2 is normally preceded by a 1st version, it's maybe the IF-C9D with a different firmware and RAM included in the 80C49C chip (instead of using the internal memory mapper as RAM disk).
that 9 pin connector (one pin from it) is going to SN75170 IC (RS-232 Interface IC Line Driver and Receiver) and from there to computer (so not going to that 80C40), so it is probably just RS232 interface?
Not sure if it corresponds to BASIC instructions, but I can see five SUPER texts in the 4000-4772 part ... I guess it's for 5 different types of superimposing.
Also the text 'VHD T11" is intriguing. Does the T means "Title"'?
Maybe just a software to add subtitles to an existing VHD? It could explain this second RS-232C interface....
Anyway, probably different from the BASIC extension in the 'normal' model (IF-C9C)...
There is those super0/super1/super2/super3/super4 basic commands. That super4 command changed the output to black (I am using currently composite output as I have not made the rgb output cable yet), other commands did not make any meaninfull thing even though the picture somehow changed as little as possible.
When comparing this C9D pcb to that C9c pcb picture (even if that is very blurry and dark), it seems both are same, but this C9D is actually stripped down version as right side of ICs are not populated (except the one chip RS232 circuit is present instead).
