I had a feeling this was going to be a learning experience for me in what exactly I have for a bike!

Problem is I literally pulled this thing out of the weeds of the seller's backyard where it sat for almost 20 years rotting. And he didn't know anything about the bike other than he wanted $100 for it.
Anyway, the reason I believe I have a '70 motor in the '78 frame is because of this serial number on the engine just above the front sprocket cover. Sort of hard to see in the picture below, but is says:
CB750E-1090125
Can you first confirm my suspicion that it is in fact a '70 motor?

Next, I can almost certainly tell that the rear sprocket is not offset by 10mm, but can you confirm by these photos?


And finally, by looking at the pictures of the two rear carriers Tree posted, it looks like my carrier is a K8 because it doesn't have those ribs. It has the smooth curves.


So what the hell do I have going on here? hahaha