Last week, Nokia unveiled to the world their first smartphones powered by Microsoft's Windows Phone operating system, namely the Nokia Lumia 800 and the Lumia 710, but it seems that these are not the only Mango handsets that the company has been already working on.

At the Design Museum in Shad, Thames, in the UK, the vendor put on display another Windows Phone device, one that was not seen before.

No specific details on what this device is all about have emerged until now, but some suggest that it could be one of the upcoming Windows Phone handsets that Nokia has in store for us.

“Nokia Products that changed the world' is an exhibition currently on at the Design Museum in Shad, Thames. Let us know what our favourite Nokia is in the Time Machine and be in with a chance to win prizes,” Nokia notes on its Facebook page, where the photo to the left can be seen.

Some suggest that this device would be the same Nokia 900 smartphone that was rumored only days before Nokia went official with its first two Windows Phones last week, and which was said to be on its way to the market as the Nokia Ace.

However, the specificationslist of the rumored Nokia 900 were strikingly resembling those of Nokia Lumia 800, and chances are that the rumor was actually about one and the same smartphone.

As for the handset that can be seen in the photo that Nokia put on its Facebook page, it does not seem to closely resemble the already made official handsets, nor the rumored Nokia 900, at least not when the outer design is involved.

Undoubtedly, Nokia will bring to the market some great new Windows Phones in the not-too-distant future, since the company already announced that it aims at launching many more new handsets powered by Microsoft's mobile OS, and chances are that slim, highly appealing phones as the one in the photo above will also be there.