I would imagine the round plug referred to is the connector for the ganged E36 (very late M42 & M44) coil pack set. The harness would of course need to accept that coupler (or just get spliced) and go to four individual connectors for the coils.
Unrelated to COP - I would love a way to adapt that ganged coil pack to the individual connectors on the M42. The only justification for this is the ready availability of those coil packs and the paucity of individual coils in the junkyards near me (and probably most everyone else).
Keep up the good work, COP FTW & etc...