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Thanks for posting the image of the front. Please review this website and the various grills known for that stamp. None fall within the range of that grill. stampsmarter.com/learning/ID_1867_1868Grills.html
Fake and counterfeit grills are known and not all that unusual. I've had more than a few myself

I'm far from an expert on the issue but can tell you that if the item is submitted to the Philatelic Foundation you'll get expert opinion and peace of mind for less than the price of a tank of gas. If in doubt, send it out.