Rubber Stopper #6 is pre-drilled to be compatible with our Econo Lock Airlock for easy fermenting of products. Use it with our Airlock and our reCap Pour Caps to turn a mason jar into a fermenting vessel. It also fits our Loop Jugs shown below under accessories. Made in the USA. Scroll down to the From the Blog section to see how to use reCap Pour Cap with Airlock & Rubber Stopper for fermenting and get a Kvass Recipe.