I have a Monterey 248 lsc with a 5.0 These plugs on my engine are only for draining the water.
Is it possible that someone may have dropped it in the bilge while they were draining the water
On the out drive there are 2 screws with washers they are used to drain the outdrive fluid. If you can take your boat check those screws
You can go to Mercurys website and in the upper right hand corner you can add your engine and look at all the parts.
http://www.mercurypartsexpress.com/us/
Tony