I was looking a bit more in detail at this strange difference in having a filter or not and later it became clear to me that Volvo Penta only issues this filter to engines connected to AquaDrives, a special tailpiece for planing motorboats. These things have the waterintake integrated in the sterndrive. This why VP puts the filter after the pump.
Nevertheless the filter is still a useful addition for inboard engines with a normal strainer.
