I have one of these on order from the ebay vendor based upon Paul's experience. I have an OEM cap with the Mercedes logo in black plastic, so thought it worth the small outlay to get a stainless steel cap whilst retaining the original in store. Certainly looks the part.
I noticed that the vendor supplies both lockable versions and non-lockable versions. If you search for 'mercedes pagode fuel cap' it will show both versions for, I think, the same price
Peter