Simple, I suggest: the long frames became mixed with the regular frames and when in the Build Station, two dimensions of grip frame showed up. Thus the "C"marked panels in one bucket and the non marked in a second bucket. based on trial and error, the builder knew which bucket to go to until the three hundred long frames were finally used up......There were no "extra hands" to sort prior to build and the bucket syndrome was born..........Look at the old photos of day one and the few workers, and you will see "no extra hands" to screw around with sorting.........Bill would have said: "mark those long panels, the bastards, with a C and ship those d**n guns!"......He was a tough taskmaster and frugal to boot..... 8)And, do not ignore that the wood boxes had to be differernt as well, or at least routed out to fit the longer frame........