Martin,
You cannot be sure, unless the reproduction Chevrolet truck ID plates are the ones commonly available. Those have a "box" for the code that is not as tall as the code-box on the original plates.
The reproduction GMC ID plates are a very good match to the original.
The other "give away" regarding after-market ID plates would be if the fake plates are installed with fake/wrong attachment hardware.
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unique style of clutch-head screws for 1950 and earlier-
special front-installable rivets (these are aluminum reproduction rivets).
