It's summertime, it's time to go with a caravan on the road for holiday

. I remember when I was a little kid and my friend had Cadillac (27c) with Caravan Bluebird(23c), how I was envious of his models. It was an great honor for me, when he lent me for a while his Caddy with the caravan. I think it was a good idea by Lesney to introduced a caravan in the production, because everybody wanted to combine their models with this caravan.
Here are a few sample of the caravan:
23b metallic lime-green with GPW ø9x20
23c metallic lime-green with GPW ø9x20
23c metallic mauve, black door, maroon base plate, with GPW ø9x20
23c metallic mauve with BPW ø9x36
23d lemon with GPW ø8x18
Which model, do you think, is the most rare one?
Antonin