Your theory sounds good to me. I just looked at the original user manual and neither of those jumpers is listed.

Do you have a CT1350B that you can test with?

yes I have but I cannot test until Monday morning... if I "disable" FM do you have any idea what can I use to test if the system sees the OPL2 or not? Maybe some adlib game?

Yeah, I would just use a game that offers a specific Adlib setting for the music and/or sound.

Those are not jumpers, but rather, output headers, intended to be connected to the corresponding mixer input headers of the uncommon SB2.0 CD-ROM interface card (CT-1800).

Oh, so if you "disable" fm for example by putting a jumper there what you actually do is grounding it's audio output, correct?
I am not even sure now if just pulling the ym3812 is good as it will leave several floating pins on the board... Thanks Cloudschatze for that rare info 😀

I just got a 1350B in the mail today and was wondering the same thing. I thought I had heard about them being output headers, but couldn't really think what they would be used for.

Documentation on these things really could stand some improvement. I still have no idea what many of the jumpers on some of my sound cards actually do.