I had another go at it and found it was an SAE fitting. It had just been banged up a little and burred. When I drove the socket on with a hammer, it worked well. I just wanted to drain the disgusting fluid out of the diff and final drives and will now re-fill it with SAE 30 before winter is upon us. I've looked and looked, but there is no tranny drain plug. I will have to assume filling the diff will fill the transmission and final drives.
Thanks for all the replies! Here's a pic of the beast. I hope to get at it next year.
