Mc16 crank cases look nothing like the later MC18-'28 cases. However cranks from the 18's will not fit into '21's and '28's or visa versa.

Telling if a crank case is from a '18, '21, or '28 is quite hard from a photo.

An '18 case doesnt have a GPS hole which is about the only external difference.

As for if its running a 300 or not... If it had a 300 top end attached to it that just means the cases are under less stress than they would be if it had a good 250 set up
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If I have to take the carbs off once more...