I agree. The extension gives it away…

That is definitely a 20" engine. I have 4 of those engines. That particular engine is a 3-cylinder looper and a real powerhouse for what it is.

No, Frank.
That engine in particular is a 500CC engine. It was originally developed by OMC for the 500CC European Race circuits.
It was also developed into the 15" and 20" domestic market in the configuration you see here. It was very expensive to manufacture and was never advertised but the dealer network knew about them and ordered them anyway. Super smooth running and tons of hole shot and top end power.
It has a 565CC big brother in the form of a 35hp as well. The Oil injection system left a bit to be desired but strip that off the engine and mix your own fuel and they are fun engines! I have a 15" version on my Gheenoe and it ran 37mph with my trolling motor battery in the bow and me in it. Runs around 31-32 with two in the boat and gear. As you can see, the engine looks like half of a V6 with a bank of cylinders missing. This was purposely done to engineer in a super efficient exhaust system. You can see the size of the exhaust system off to the right of the cylinders. Optical ignition as well.