What would "S&D" stand for on a shelving EQ? For instance on the Olympic Studio1 console EQ

[InterstellarObscure]

Not sure if it's the right forum for my question, but was looking at a photo showing the Olympic, Studio 1 console EQ, and hoping to satisfy my curiosity about it. This might be the only close up photo showing the studio1 module; found here on the forums. The studio1 module is on the far left, with the silver-skirted knobs:

First there's the input gain switch and then a high frequency cut/boost switch (maybe it's 10kHz like the Helios beside it).
Third control is a "Fine Gain" pot, not sure what that's for.
Fourth control appears to be a two position switch for "Nor" or "S & D". Is it related to the Mid-Frequency Peak/Trough switch on later Helios EQs?
Last is the Low Frequency cut/boost at 250 or 60Hz.

I've always been intrigued by this partially-Germanium Olympic console, being used to record so many classic records, and apparently being a favorite among many musicians of that era.

Do you have an idea of what these controls might be doing?

 

[k brown]

Not sure how it might apply to an EQ, but my first thought was "Normal" (as in normal stereo), and "Sum & Difference", possibly for EQing a Mid-Side recording?.

https://www.uaudio.com/blog/mid-sid...yNBtC0x-X_5X_tlYQZb4KlEY4L6fV-jiPfJfWJ2VapAvS

Is it a stereo unit?