The issue is now solved (almost), and it was the 2702BM set itself. I checked polarity within the 6041G separately mounted key, both with a DC voltmeter and a polarity tester (double kill), and each and every line was good. Still was a mystery why lines 2-4 were okay but lines 1 and 5 weren't with regard to polarity at the 2702BM set.
I took another 2702BM and modified it for 1A2 service in the same manner, and then cabled it into the separately mounted key, and it all worked fine. My goal was to have a beige non-key set with the corresponding beige 6041G separately mounted key with a 6-conductor beige cord. I could have swapped the covers and handsets between the two 2702BM sets, but it was easier to find a 702B in beige and use it, which is what I did.
The defective 2702BM is still a mystery. I've traced its wiring twice, but I didn't find any issues. Next step will be to unwire it and rewire it completely, testing all components along the way. No telling what happened to it during all the past years. I once had an issue with a multiline set with a headset jack, which didn't work properly. It took a complete rewire and continuity check on each lead to find the lead with a broken wire inside the vinyl. Maybe this one is destined for that, but on another day.
Thanks for all the help.
Bill Bentsen
