By legal definition all work is copywritten. There doesn't have to be marks or statements on the work. It is illegal to steal someone else's work and claim it as yours … or put it in open source. As has been mentioned, computer program languages are often open source, but the products produced from them are not. For example, anyone can buy MSExcel, VB or VB.net and use it. But the programs produced by it can often be "copywritten" and/or company proprietary. Changing a part or two doesn't make it a new design. (sarcastically) Ask Garmin or any other manufacturer for there code … it's written in some language that's publicly available.

BTW, Paul Poberezny BOUGHT the rights to the Corbin airplanes (drawings and part patterns). How do I know? I did the drawings to document, update and modify them … and widen the Super and Junior Ace airplanes. Are there errors? Yes. The cabane measurements don't match the number on the general arrangement drawing as an example.

Several have mentioned talking to the original designer. Great idea. They are normally open, but be prepared to live with his/her answer.