I think JimB is right on. My limited understnading is that they vacumm sucked the sap out of the wood, kiln dried it, then put the wood in a pressured vat with rosen and dye. to refill the voids left by the sap.
I don't know if the pressure treatment was before or after the riser handle was formed. I would think after, because the risen is only going to go in so deep. If they pressure filled it before forming the riser handle, seems to me that the grip section might not have any rosen in it.
JimB says he is no expert, but he probably is more of one than me.
Just my thoughts.