My guess would be the wrong head is missing a mounting bolt threaded hole for something, or the existing bolt hole is in the wrong location.

I'd see if whatever bolts onto the heads can be shifted, or holes elongated. If one head is interchangeable, and the other isn't, its has to be because of a mounting bracket, or a clearance issue for something added to the motor. I'd find it hard to believe the wrong head wouldn't function on the motor, but these days, anything is possible. Gene