“ New injector is a Beck/Arnley 158-0438. They claim it will work and be compatible with the existing injectors. My confusion is how an injector with three orifices is compatible with those which have but one orifice. Unless the B/A part was manufactured as a replacement for the original equipment.”

Irregardless of your observation on the number of orfices…
That’s the correct injector for this engine. It will run just fine. Don’t think zebras when you hear hoof beats. Having said that, put it in and see what it does. Yes the BA injector is an exact replacement for the original Bosch injectors. If you’re stuck on having original Bosch injectors, you can get one from Iroll Motors.