The investigators said that based on the written numbers, the certificate was fake. They used what they said was the 1961 manual to prove the written numbers didn’t match what was typed in. Now if it turned out it was a 1960 manual instead of 1961 but was still valid, that would be a minor point, slightly embarrassing but not affecting the conclusion. (Although you can bet the libs and lawyers would be all over any minor error.)
However, when the manual they are claiming proves the mismatch is actually from years later, and a totally different numbering system was in effect at the time, it shows they were not using due care with their proof. Your Ferrari analogy is not applicable - it’s more like an official investigation claiming you broke a 1975 law when your alleged lawbreaking occurred in before the law was in effect in 1970.
To repeat, it makes them look foolish and careless. It’s not a matter of “interpretation” it ‘s a matter of whether the material they claim they have is actually what they do have.