Tamiya make two M4A3 kits at the moment (the "A3" part designates the Ford v8 engine, and hence the engine grille part that we're interested in), but they both have four rivets. This appears to be the right amount of rivets for the real M4A3 tank, but the wrong amount of rivets for the X-wing.
The kit I got - with five rivets - is an old one from the 60s/70s - it's a 1/35 motorized kit of the M4A3E8 (the "E8" part designates the horizontal suspension bogeys). MT218 (or maybe MT218-600?) is the kit number. Looks like they come up on eBay pretty regularly - there just happened not to be any on the day that I found mine, and splurged on it. Just search for "tamiya sherman" and it should be obvious from the description whether it's the right kit or not.
Edit: Just to clarify, Steve PM'd me about this kit just after I found the one I bought, and of course I'd never have found it if he hadn't pointed out what the part was, so it's 100% Steve's find.