It was released alongside the Summer Sideshow, alongisde Community Weapon Pack 2 and Golden Weapon Pack 2. There was also a Steampunk Mrs. Foster made available to unlock during that same Summer Sideshow event, by completing achievements related to the new map, Lockheart's Steamland.
"Mr. Foster has been a firm favorite with Killing Floor fans for years now - but he's been all on his own. So we thought it was about time we introduced you all to... Mrs. Foster!"