I am not familiar with the production process of these but I wonder if its a quality control stamp of sorts. Maybe to show that they picked up one toy, examined it to make sure they were being put together properly, and then boxed it up with the rest of the completed toys.
Anyone have an AST-5 who would be able to look to see if its got a stamp as well. I will have to check my rebo piano and see if its got a stamp on the bottom.