The Ceramic Headphones were made specially for me by a manufacturer. As with anything special made and not purchasing 10 thousand or more of them at a time the cost was high.
These are High Impedance Ceramic Headphones. They have a DC resistance of over 20 Million Ohms. Their tested impedance is about 10,000 ohms. They have 1/4 inch mono plug ends and their cord is about 40 inches long. These are very good Crystal Radio Headphones.
The Piezoelectric High Impedance Ceramic Headphone is mono and has a 1/4 inch end plug.
Please note that if these are not operated from a transformer they need a resistor across the headphone input. Most crystal radios have a resistor in the audio output, if not add a 47K to 100K resistor in parallel.