My stock RBC inlet on my 06 Si is 64mm. Measured with a digital caliper. Not sure where you got your general knowledge.Is a general knowledge and proved multiple times that the stock RBC inlet size is 62mm.
I remember doing this test of using the TB gasket as the comparison tool between my J35 outlet and stock RBC inlet. The difference was clearly. The RBC inlet was smaller.
Check with a caliper to get a more accurate reading of the size.