Assuming they are fairly fine, it would be the normal wear between the starter gear and the flywheel. Larger pieces of metal may be parts of the flywheel teeth.
Prior to just going for a starter replacement, I would verify that the solenoid is working correctly and delivering power to the starter. This also assumes that the cables are good (no corrosion, and they are significantly not heating up), the connections are clean and tight, and of course the battery is good. Being you have a '91, I would tend to think it is in fact the starter. I replaced mine ('88) recently as well with the Arco unit. However, I've read on here where some folks have had sections of the flywheel where the teeth are missing and the starter could not engage. You could rotate the engine manually and inspect the flywheel for this.
Most auto parts stores will load test starters for free. Also, the starter is one of the items on a boat that is required to be marine grade.
Many on here have 'upgraded' to an Arco gear reduction type starter (spins faster). They cost a bit more, but worth it in my opinion. The link below is to their website's tech tips, click on the "How to determine starter rotation" if you need help figuring out CW or CCW for your application.
Yours should be the 70200 unit (verify) on page 33 below, for a Ford 351W.
Several websites carry them, around $125 or so.