The Sound Blaster 2 has a stereo audio connector with the mono output from the adlib and PCM split to both left and right. The separated left and right are then combined with the left and right of the CMS output to produce the final result.
You hear it on both channels both ways but without the CMS it's just the same mono on both L and R.
The PCB says CT1350B, but all the cards I have seen also have a sticker on the reverse that says CT1350A. Why is this?
Creative being Creative. Unless someone here worked at Creative and was personally involved in that, the particulars of exactly why are probably lost to history.
I didn't actually notice that until just now, or at least if I did I forgot about it. I got up to check my SB2 and it's there.