Re: Mystery Part ??
That's a rubber plug used to cover a nipple when the powerhead is being painted to keep paint off of & out of the nipple. If the powerhead was built and painted and then shipped, the rebuilder leaves the plugs on to keep the nipple protected from dirt, dust etc during shipping. Sometimes a rebuilder will tape up the nipple for painting and then put a plug on for shipping.
Different manufactures and remauufacturers use different ones. That one looks like the ones OMC used to use. Silver indicates that that one came from something being painted Sea Drive silver. Someone may have just used it to temporarily plug something on your motor.
At any rate, if that was covering something with a hole that big and now that nipple is open, you'd know it. If your motor is running fine, don't worry about it.