well unfortunately i can't help solve the mystery, my 88 16v sensor is a 4-wire heated one. and, unfortunately, the 16v has two solid black wires among the 4. i have an 88 BGB, so i know what the 16v wires are. one for signal to the ecu, one for signal from the ecu to turn on and off the heater, one power, and one ground. but i don't know that the ones on the 20v are entirely. i know which is the heater signal, but then it shows two more of them going through the ecu?
oh, and i think you can use a 1 wire with a 4 wire setup. you just forego the benefits of a heated sensor. thats all. the ecu still gets its signal just fine. i think some mk2 guys do this when dropping turbo motors (4wire) into n/a cars (1wire).
and unfortunately i can't weld, so just putting another bung on isn't as easy as it sounds. (wish i could tho. buying a welder and learning how is right at the top of my list for things to do in the next few months.)