We came across this same issue on our EAA Chapter 105 RV-12 iS group build. There are two versions of this part, and we had the wrong ones in our kit. On page 9iS/U-07 of the KAI in step 5, it references HS-01231C-1 we received HS-01231C in our kit (NOT with the -1). A call to Van's and a parts run confirmed we had the wrong ones. If I remember correctly, the parts are slightly different in shape but most importantly they have different hole patterns. I would call Van's service people to see what they have to say. There should be four of these gussets with the same part number.
Hope this helps.