'88 KX has an eyelet on the bottom, rather than the fork/clevis of the 89+ bikes. If you swap to an '89+ KX500 linkage (or '89 KX125/250 linkage), then this shock will fit into an 88 KX.
The same is true for the '87 models. Not sure about the '86 KX125s - specifically, I don't know whether the later linkage fits, and whether there's issues with the remote reservoir.
Definitely won't fit into an 86 250/500.