Looking through RealOEM, there seems to be an old version of the OEM skid plate which is NOT compatible with the later version.
Old PN 51-71-1-929-451, from 09/84 - 08/87
New PN 51-71-1-971-001, from 09/87 forward
So, I guess if you can track down the 451 part, it may fit. I found a photo of it online. It is distinctly different than the later style ones, so at least you should be able to identify the right one if you can find one.

Here's the newer one:
