Chaterlea25,
It would appear you hit the nail on the head:
The old valves are as you said, the inlet were magnetic, the exhaust were not.
Have been given a set of hardly used valves, they are all non magnetic, two have a bigger, higher and rolled lip on the mating face, which I will use as exhaust, the only other difference I could find is the distance between the top of the collet recess to the top of the valve was slightly different, also agreeing with your statement (the exhaust valve was slightly longer).
Many thanks,
Josh
The one on the right is the exhaust.