Testors had 3 series of models, Freshman, Jr and sr. Each one was offered in 3 sizes,09, 19, and 29. They were a progressive series of models with each one being a little more complex in the build to take you from a rank beginner to a decent flyer and builder. The Testors company withdrew them from productions suddenly and without any explanation in the early 50's. Kits are rare and the smaller ones particularly the 09 series seem to be the hardest to find.
I was the lucky receipient of a Senior 09 kit recently. A gift from a friend.
Dennis