Bolt identification request

[motofish]

I've recently done a lot of work on my 2004 Yamaha 150 and its now running great. Trouble is, I discovered a loose bolt in the cargo pocket of pants that I was wearing during some of these sessions (which also involved lawnmower repair....).

Does anyone recognize this bolt? Its unusual because of the very shiny rounded surface on the top. It fits a 12mm socket. 
imgur.com/a/gwI9zhV

1) Oil change
2) Fuel injector swap
3) VST and fuel filter replacement
4) Impellor replacement

I will appreciate any replies on this for my peace of mind.....

[Juan]

Assuming your lawnmower is like mine and is S.A.E., and since the bolt is metric, I'd guess it's from the Yamaha. Could it be one of the bolts from the lower unit or maybe the one for the rudder anode?

BTW: If you can use them, I have the owner's manual and a new oil filter for that motor for the cost of shipping.

[freezerfiller]

Looks like the last few threads are stripped. Looks like a lawnmower bolt to me. Is it stainless?

[motofish]

I removed the rudder off the lower unit to confirm that I did replace the bolts. All good there (photo taken of the bolt head in case anyone in the future wants to know).

The stainless question was key. I used a magnet that I had scavenged from an old hard drive and compared the magnetism of the loose bolt to a set of known stainless screws. The bolt was much more attracted to the magnet than the screws, which I believe indicates that it is not stainless. And likely not a Yamaha component. Looks like I'm taking the boat out worry-free tomorrow.

Much appreciated.