Here we have five different Boch units. The back row left to right are the unmarked, the U-928A Big Boch and the incused marked with the Boch Patent date. The front row are the two tougher pieces. The white dry process with the EMBOSSED (raised) "BOCH INSULATOR PAT APPD FOR" in a ring around the shoulder and the later brown unit with the incuse patent date info.