For all these years we have believed the square end frames came out in 1938 but now we know that they were in the stores by Christmas of 1937. This also reinforces a long held belief of mine - that the red litho frames (RLF) were the first of the square end frames. The litho design was probably discontinued due to registration problems. Registration is the process whereby the punch press is calibrated to punch the flat frame out of the sheet exactly where the design is. Anyone who has ever done any silk-screening is aware of the pain that registration can be. Many parts can be wasted trying to get it just right. When you consider that multiple frames were lithographed on the sheet the problems are compounded. Now, besides registration you have to get the indexing just right. Indexing is the process where the press is calibrated to cut subsequent pieces at exactly the right spots. What a nightmare it must have been trying to get it just right.
Now consider the black frame. No lithography to worry about. Instead of sheets of steel the frames were probably made from a coil of precolored
steel. One frame after another, probably with progressive dies, no worrying about registration, and indexing is much simpler too! I bet the line
foreman consumed copious amounts of adult beverages celebrating the demise of the red litho frame!