Interesting, mine must be a vanilla, first rev. Spectrum/24 . . .

http://68kmla.org/forums/viewtopic.php?f=16&t=18863&start=0

. . . control panel for 2.07 looks like it may do the trick. What's the copyright date on PCB and ROM?

My guess is that SuperVideo 3.1 is for the later Spectrum/24 Series IV and V cards from the looks of the info on that screen.

My Spectrum/24 and the Series II cards aren't even mentioned on LEM.