Dave,
Your mystery tank appears to be a -56 or -56A. They are virtually the same tank with the slight angle change toward the centerline of the frame of the oil feed fitting that Jerry R refers to. That change was necessary to move the feedline inward away from the now relocated rear exhast pipe on the overhead valve Sportster.
So here's my theory on why a new tank was needed for 56.
The new for 56 frame featured a raised neck which in essence lowered the rest of the frame. This in turn required a change in the location of the swingarm attachment/pivot point on the lower rear frame casting. Compare a 52-55 K frame with a 56 up frame. That hole is slightly but significantly higher on the later frames (thank you David B). I suppose it was to keep things relatively level ? That change in geometry lead to chain interference with the rear of the oil tank, hence the extra relief in the recessed area. The chain guard was shortened for the same reason. The lowered 56 frame also gave us the two bend jiffy stand.
Feel free to pick any or all of this apart. I have been accused more than a couple times of talking out of my ass.
Jack