Thru Stanchion
My '94 270 is vented thru lifeline stanchion, similar to water tank on port side, on my boat if you remove cabinets you can access vent hoses. I believe this venting is shown in owners manuals posted at national association website. I must flush head well with water and pump bowl dry leaving valve in dry position to minimize odors in head. These are webpages from Catalina Direct showing venting Stanchions.http://www.catalinadirect.com/stanchions/stanchions.htmlhttp://www.catalinadirect.com/stanchions/stanchion%20vent%20a%20base.html