I found the answer to my own question after looking at lots of parts pages. It appears that SX-A drives had a tiller cover integral with the transom shield and that part of the shield had two studs that passed through the upper two holes in the inner transom plate. That inner transom plate has remained the same throughout SX variants. Starting with the SX-C the transom shield was modified to add a removable cover over the tiller and this required the elimination of the top two studs. So, if you have a removable tiller cover the top two holes on the inner transom plate should be empty.
I guess it is strong enough with out them! I haven't read of any failures so I'll stop worrying about it.