Okay I took the three screws out of the top. They seem to be brass, a magnet won't pick them up.Does the top now screw off? Or just pull off? I don't want to get too "physical" with it . . . . .
The top should just pull off but they can be hard to get off and back on. A little oil soaked in and a twist like loosening a jar lid can help. Suggest you mark the flask body and top with tape so you can put the top back on with the original screw hole alignment.