on the bottom of the VB where the hole for the filter is, theres a flat boss that would seal up to the filter. if youre familiar with valve bodies, you recognize that the rest of it has this slightly raised grid across the face of it. that second hole in the extension catches the edge of some of those ribs. theres also a deep squiggly grove in the VB thats runs about 5" right up to that area where the second hole is. you really have to see it to understand it.

If the exyension I have isnt deep enough, i might just try to use a gasket to block it off