It's a Cox Babe-Bee .049. Extremely common 1/2A class engine. I think they are still in production by Estes in Colorado. If the tank has the words "Thimble Drome' on it, then it is a pretty early model. The words were dropped later, can't say exactly when.

Have a look here for more info on Cox engines:

http://www.mh-aerotools.de/airfoils/cox_frameset.htm

Click on the button marked "Bees".