Hi,
Good question but I don't know.
Looking at the parts diagrams, it doesn't look like anything has changed over the years but I no longer have current info.
In the link below, the depiction clearly shows the case looking very similar to your photo but without the hole in the forward area of the case.
Buy OEM Parts for Honda Outboard 5HP Gear Case Assy. Diagram
www.boats.net
In the parts depiction link below they show the drain screw down low on the case and two fill screws with gaskets located well above the area in your picture. (Items 17 and 24 times 3).
There are no other components listed like plugs or screws for the side of the case.
Buy OEM Parts for Honda Outboard 5HP Extension Case + Gear Case Diagram
www.boats.net
This is a mystery that you may need to take to a Honda dealer to get the correct answer.
In the meantime though, to protect your transmission, you may want to drain the case and see if any water has entered.
You don't say what year it is but, even if it's a second hand purchase, if it's under 5 years old it should still be under the factory warranty.
Good luck.