I could have sworn I use my big ''crankshaft slash trailer ball'' specific socket to remove that fastener. (Mine's SAE like it's supposed to be though

) A big cresent wrench or channel locks will spin that big nut off, it's not that tight and it better not have rusted threads!!
Under the sammich adapter is the two bolts you need to extract. The heads aren't 10mm though, those bolts are SAE.