This is not a mandatory item but it looks good. It stays tight like it should. I have a different brand plug for the trans side and I have issues keeping it snug to the point where I have lost fluid during a ride. This one is the way to go.
This is a good replacement for the plastic plug. I wish it had a thumb handle like the plastic plug but other than that I think it is very nice. The price is a bit high for little piece of aluminum. It fit perfectly.
I do know the oil filler cap on some Honda's 4strokes is the same for the Transmission. The easiest way to figure out if that's your case is to try your oil plug on the transmission plug. If it fits than you can order 2 oil filler plugs, one for the oil and the other for transmission.