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Please see the last picture in the gallery. There is drawing showing the difference between common 28BYJ-48 and the one with four mounting holes.

There is also sobo84's comment from 28 August with link to Aliexpress.

If you have such motors you will have to print revised models. Judging by the way you are asking, you do not have this problem so it is only a matter of fan installation - if you want to cool the steppers or not.