Hi Dave.
Sorry I missed your post.
Not exactly sure when they came out but all of the Propert caravans had them from the early 50's onwards, and I havnt really noticed anything earlier with a plastic sink.
Hope this helps.
Cheers.
Richard.
ourtouringpast.com THE vintage caravan restoration website
and home of The National Caravan Museum.
Judging by small things like Phillip's head screws that hold the alloy trim around the sink (the rest are slot head screws ) I hazard a guess its been retro fitted.