Edit: I've just realised I had uploaded the wrong one - it's actually the attachment set - download this one.
I noticed that the Knights of Montesa upgrade has a shield wrapped around the head of one of the units - I think this is actually a leftover issue from SS5.1.
So here is the fixed texture:
Attachment 11623
extract it into the following directory:
xx\Stainless_Steel_6\data\unit_models\attachmentsets
KK if you see this maybe you can put it into the next patch.