Curious... what would be ruined & how did you come to that conclusion?
(or maybe you're just being silly like I am too often.)
It is quite possible it has some effect on the front chs. As you can see on pg 2 it shows an input (probably tape out from a receiver) and a front ch output (probably going to the tape in on the same receiver). I doubt that it does much to the front chs, but if QM=EV4 there is some in phase blending involved.
Again it says the decoder "picks up" reverberant & indirect sounds & says nothing about artificially creating reverb or delay. I'm familiar with ye olde spring reverbs & that little metal box is far too small to contain one.
I read the the document provided by "par4ken" on quadraphonic system and on most QM decoding equations including Dynaquad and Dolby, the following apply:
Dynaquad: Front left = left and Front right = right, then the equation for the Rear channels have matrix equations Rear left = .86L - .5R and Rb = -.5L + .86R
QM dolby: Left = l, Right= r, Front = (.71l + .71r), Rear = delay (.71j - .71rj)
(there is a delay on the back channel)
With those equations, there is no decoding of the Front channels, only the Rear.
The instructions are straight forward in the manual: All you need is a SC-410 and 2 speakers.
The SC-410 have effects selections, those effects include: Hall, Studio, Surround and Stage. These all involve reverb and delays to mimic the sound caracteristics of various listening places. We have these settings on every 5.1, 6.1 and 7.1 modern multichannel decoders.
So the little box certainly contains the effects module or circuits for 2 reasons:
1- if it's mechanical, it must be protected against shock and electronic noises.
2- if it's electronic, which i doubt, it must be well shielded as early electronic reverb devices where rather noisy.
I make all my assumptions on 40 years of experience in electronics and sound systems and i'm still learning.