I have the NRD-515 with the matching memory unit and speaker. The previous owner installed the optional bandwidth filter, headphone jack and memory back up battery pack. This robust radio is built like a brick and is as close to 'boatanchor' as a Japanese radio could get. I just love the feel and capability of this receiver.
The later NRD/NSD series units may have had more bells and whistles, but the 515 is a legend. They hold their value well as the asking prices are quite high.
I have been on the JRC archives website hosted by qth.net, but it has been quiet lately. Maybe someone there has scanned a service manual?