It sounds to me like a typical Haas delay implementation, which is essentially a virtual doubling of a mono signal (think vocal doubling without the hassles), with some checks and balances to deal with the phase cancellation created by the time offset. The M/S thing is the balance control between the additive (L+R) and subtractive (L-R) signals.
It was put in vogue in the late 80s by some of the pricier yamaha rack effects (I forget which ones), and is still used. Think of it as a “dryverb” or, as you say, alternative comb effect. By modulating DPTH and WDTH you can also create a bit of a bulging (as opposed to sweeping) chorus.
3 Likes