Here are some drawings that may help. Be sure to scroll to the second set. I'm not really sure what the first set is.
Just an aside - the first set of drawings is for the stock bench seat. Most 150's have the optional 'bucket' seats for the pilot and copilot, but a bench seat is the base standard.
If you ever have the inner plastic panels removed from the cockpit (perhaps during an annual inspection of the aileron cables), you will notice a series of four holes arranged horizontally, about halfway up the door-post. This is where the seat-back for the bench seat would lock into place. The tilt of the seat-back is adjustable by locking it into one of the four holes.
