Jaco
Yes you are correct there is only one master speaker and it has an M lead connected to it for comms- if your front left or right speaker is the master then the other speaker connected to it is a slave connected with an S lead. All the other speakers are slaves and are connected with S leads in the current 861v4 system. On the 861v4 since it has only SL outs you designate one speaker or pair as Master by connecting to the right side of the SL connector. You probably know all this but just in case.
The M leads WILL NOT WORK for slave speakers even though the 180 connector on the opposite end of the M cable looks the same as an S lead connector because of the pin configuration ( there is one extra pin soldered on the S lead cables) and that allows the communication to happen between the speakers. So if you cut the 240 DIN end of the M lead and replace it with a phono jack attached to a BNC connector it will not work. You need to use an S lead cable and cut the end off of it and add the BNC OR simply add a 511 slave patch. Again I'm not sure about a mixed SL with nonSL setup but for a non SL system the above works.
Regards
Joe

My gear: Trinnov CI, 271, 8000.1(SEs), 7200.1 HC (SE), 320s, 3200s, Seaton F18 subs, MS200 (2nd zone), 218, 210.
Other: JVC RS-4100 projector, Lumagen Pro Radiance, Panamorph DCR anamorphic lens, Stewart G4 130 screen, Kaleidescape Strato K, Rvolution 4K player and Zapitti NAS, Oppo BDP-203,Roon Nucleus