The cases, which have arrived at an AT&T retail shop in the US, show a wider device than the iPhone 4, with a tapered (some might say 'raindrop') design.
The only other thing we can see is different from the iPhone 4 is that the mute button seems to be on the opposite side of the handset.
One other detail worth noting is that the sleeve's packaging calls the product "Silicone sleeve Apple iPhone 5" - you'll note that it doesn't say iPhone 4S. But then you'll also note that the name is printed on a dodgy-looking white label.
We've spoken to more legit case manufacturers like CaseMate and Griffin in the past and have consistently been told that, while they use speculation and guess work to get most of the case design done in advance, accessory companies find out what the dimensions of new iPhones are when the rest of the world does.
Case makers usually just tweak their accessory designs on the night of the iPhone launch and start manufacturing right away.
Given that Apple's iPhone event isn't until next week, we're inclined to think that this is a prototype model errantly shipped to AT&T, an anonymous case maker taking a punt, a fake or a PR ploy.
That doesn't mean that the new iPhone won't be called the iPhone 5, nor that the tapered design won't come to pass – plenty of other case makers have hedged their bets with this new look – but we'll take a pinch of salt with these particular silicone iPhone 5 sleeves, thanks.
While away the remaining hours to Tuesday's iPhone 5 event with a look at our feature wishlist for Apple's next lust-worthy handset:
Get daily insight, inspiration and deals in your inbox
Get the hottest deals available in your inbox plus news, reviews, opinion, analysis and more from the TechRadar team.
Former UK News Editor for TechRadar, it was a perpetual challenge among the TechRadar staff to send Kate (Twitter, Google+) a link to something interesting on the internet that she hasn't already seen. As TechRadar's News Editor (UK), she was constantly on the hunt for top news and intriguing stories to feed your gadget lust. Kate now enjoys life as a renowned music critic – her words can be found in the i Paper, Guardian, GQ, Metro, Evening Standard and Time Out, and she's also the author of 'Amy Winehouse', a biography of the soul star.