The inner skirt to these two U-928 Thomas Boch patent insulators are different. Note that the one on the left is phallic shaped and the one on the right is cone shaped. Since Thomas was not able to thread wet process porcelain at this time (circa 1898-1900), the threaded center part was made of dry process porcelain. Even though the Boch patent was for glazefilling to fill all the area between the shells with glaze, note there are air gaps where the glaze did not fill in.