this style is called the double channel,,and it was claimed to be indestructable, it came in 2 other sizes ,,3 & 1/4,, 4 & 3/4 and yours,,i believe this to be a copy of the joseph lucas design which was around before king dick, so probably earlier than the other styles..the other more typical design came in 2 shapes almost identical,,(so dont think you got a set till you got 4 standard and 2 ideal),,there may be more sizes not in my book...of course then you have the auto adjustable style,,and later the crescent style.
most of this info comes from my 1947 catalogue,,so there may be more variants to add
