Ok here are some pictures of the two different track arms to try and tell the differences.
Beside the differences in the shape of the arm, maybe the monoball/bushing end of the arm has stronger internals? By hand I can't get either of the bushing ends to flex so I can't tell that way.
Here are some photos showing the differences.
From this side they have the same outline, but the 987 part has more "ridges" for lack of a better word, where the 986 part is flatter in the middle with no raised parts (and I don't have a photo of the other side so I don't know if it is the same, I will check tonight)
Here are two additional shots showing the end detail. The holes line up exactly, I got some M14 bolts to test this out and you can see them in these two photos.
This is a detail of the part numbers with some part (the triangle?) ground off.
This is a top down shot, with the 987 part on top, you can see how the curve of the arm is different, the bottom arm (986) sticks out toward the top of the picture.
This shot is with the arms turned over so the 986 arm is on top, showing the curve from the other side.
These next two show the arms side by side, the arm with the "V" sharpie looking mark is the 987 arm.
Maybe over Christmas break I will get a chance to install these and I might have more feedback then.
Steve