Spotted what appears to be a very early 'S' on ebay which has been listed for parts only. It states that the date of registration is 1/5/71. Might be worth a look if your local to it. Sorry, but I haven't got a clue how to post a link, but it is listed under classic cars - Rover, item no:160589477886. If anybody goes to have a look & it is what it says it is then tell us all about it, especially the chassis number.
Roly.
Roly.