The 1/4" bar is a "0" size powder bar, and the others are "00" powder bars. Early Star Progressives had "0" size powder slide housings which Star later discontinued in favor of the "00" size powder slide housings. The size should be cast into the side of the housing...though very early models may not have a designation. Star offered spacers to convert "0" housings to "00" housings, as well as spacers to convert "00" housings to 1/2" and 5/8" housings.