That is what I though it was. I have the same weedwhacker. But, it is new and my wife might not appricate ripping it apart. But trust me, whe the rest of it wears out, it will be my next project. Mine has the same oil fill hole in the back. You are right, changing the oil might be an issue.
I don't think mounting it inverted is an option. That would put the oil on the back side of the piston. That will not work. Maybe for a short peiod of time (loop, roll, etc.) but extended flight is out.