The idea is that the shape of fragments is different for different missiles, that's how the hacked report pointed to a missile used by Buk M1 and not any other variants.
Russian media augment this letter with Ukrainian intelligence guy saying basically the same things to a reporter on Ukrainian TV.
It all looked perfectly okay from Ukrainian POV - it was a Buk, not a fighter jet cannon that brought down Boeing - until Russians pointed out that only Ukraine has Buk M1s in the area, about 70 of them.
Another thing is that this fragment should be in custody of Dutch investigators but Ukrainians are holding onto it and keeping it secret. That is, of course, if the hacked letter and images are legitimate. So far Ukraine hasn't commented on it officially. There's no reaction from Holland or Malaysia either.