I am not sure what you mean. The GE Monitor Top refrigerator was introduced in the 1920s and produced until 1936. It was not cast iron but porcelain over steel.
The original refrigerant was Methyl formate, but some people have updated the Monitor for use by adding modern coils to the back and using an R12 blend of refrigerants. That way, the old appliance can be used.
The value of this wonderful old kitchen appliance depends on its condition. Damage or missing parts subtract from its value. Replacement parts are hard to find. Values are all over the place depending on condition. They can go from $275.00 to close to $4,000.00.