For sale is a Hitachi AT-100 Ultrasonic Transmitter/Receiver that was specifically made for the TM 500 series. These are very hard to find and I have yet to see another set online in years. These are all working modules, please note that the TM506A Picture is NOT involved in the purchase, it was just used to test the modules. 

https://w140.com/tekwiki/wiki/Hitachi_AT100#Key_Specifications