Twelve-cylinder Ferrari roadsters are hardly what you might call "common," by any stretch of the imagination. Until the new 812 GTS came along, Maranello hadn't put them into "standard production" in decades. But even in such rarified company, some are rarer than others. And this unconventionally sexy beast right here certainly qualifies.

What we have here is an exceedingly rare Ferrari 550 Barchetta rebodied by Zagato in the last collaboration between two storied Italian houses. It's one of only five made, and it's coming up for auction next month at Bonhams' New Bond Street headquarters in London.

The story of this rare machine starts with the 575 GTZ that Zagato revealed in 2006, and of which it ultimately built six examples. Following the coupe, the storied Milanese carrozzeria set about making another five roadsters. But instead of basing it on the newer Superamerica, Zagato reached further back into Ferrari's modern history and sourced five pristine examples of the rarer 550 Barchetta Pininfarina, recrafting them in similar style to the fixed-roof 575 GTZ. Each carried a sticker price of £1 million – about $1.3 million in today's money, or $1.6 million at the time.

Given the original asking price and how few of them were built, we're a little surprised to see that Bonhams estimates this example will sell for only £500- 700k, or about $770k (give or take) at current exchange rates. But then it wouldn't be the first time that Bonhams has undervalued such a rare European exotic.

Less than two months ago, the same auction house handled the sale of a Koenigsegg One:1 (one of only six made) with a pre-sale estimate of $1.8-2.3 million. The manufacturer was surprised at how low Bonhams valued it, and Koenigsegg was ultimately vindicated when the car sold for more than twice that at over $4.6 million.