From the looks of things you're right, it is the 1951 style mount, although with all that aged protective coating (hahaha) stuff it makes it a little hard to see for sure.
My thought on why you have one bolt up and one down seems that someone has already changed it out once before (???) and didn't put it together the way it was originally installed. It could be that whoever changed it before did what you are thinking and changed it without removing the balancer. I could see that as a possibility, but have no idea if that does work or not as I haven't attempted to replace my front mount yet. I also agree from initial appearances that the outer bolts seem upside down. Maybe more will be revealed after some cleaning to expose what is there.
Let us know how things go.
