Of course there was a Dual CS 505-1.

Here's a link to an image of a genuine service manual cover: http://elektrotanya.com/PREVIEWS/634...05-1.pdf_1.png
Here's another link to further info: DUAL 505-1 Turntable
Here's a video clearly showing CS 505-1 on its model ID sticker and 505-1 on its serial number sticker: https://www.youtube.com/watch?v=zWOt5UNFKds

Here's a link to a 505/506-1 service information sheet dated 1980: http://elektrotanya.com/PREVIEWS/634..._sch.pdf_1.png

Here's a link to an image for a CS 505 instruction manual: http://dual.pytalhost.eu/505/cs505_1.jpg

Printing errors aside, it may not be that far-fetched to assume there are worldwide variations which include models: 505, CS 505, CS 505-1, CS 505-2, etc.
Whatever the accuracy of this assumption, it's an inescapable fact you couldn't walk down Tottenham Court Road without tripping over a CS 505-1.