Usually one main bearing (Typically the center or the rear main) has a thrust surface that limits how far the Crankshaft can slide front to back in the block. There is a spec for this. I have no idea what it is for an FE. I always used a dial indicator on the front of the crank and slid if front to back. It's been too long to remember, but 0.010" seems to be in the ball park. How far is your crank sliding? The picture is not enough for me to say anything for certain. I am just pointing you to check the front to back play in the crank, and compare that to the specs. I do not want to tell you something wrong.
I found this:
Paul From South Orange, New Jersey Asks
http://www.4secondsflat.com/Thrust_b..._failures.html
If you do have a bronze distributor gear, certainly pull the distributor and check it. It would also be worth pulling the valve covers and looking at the guides. Do your rocker arms have brass bushings? Look at any part that is made from the metal that you found.
I found this article written by Barry:
http://diyford.com/ultimate-ford-fe-...nkshaft-guide/
This paragraph states the thrust is on the 3rd main. I didn't find and axial play spec, but it may be in the article, as I skimmed through.
"The thrust bearing in an FE is on the number-3 main. Folding a bearing into the proper shape during the manufacturing process forms the thrust bearings. If the bearing is folded too far, or not far enough, it will not have proper thrust contact. This is the reason for working the crankshaft back and forth a few times during assembly; it helps to form the bearing and correct variances in thrust-face angles."