It is proably not possible to be sure it is a fake or not but I have never seen another one like it. Would AMG have altered their logo if a customer paid for it to be a certain style?
As for the lack of tacho, I wouldn't rule it out because of that. Just as someone could have put a full AMG kit on a 280S, someone may have had the AMG speedo available and used it on the only car they had. Perhaps it was within their budget.

I would be surprised if it was AMG fitted though.