One thing to keep in mind is when was the manual printed?
The original Kodacolor roll film introduced in 1942 only had 6 usable exposures per roll of 120/620. This was because the last two exposures on the roll were used for color correction at the processing lab. If this is what your manual is referring to, that would make sense.
I'm not exactly sure when Kodak changed Kodacolor roll films to the correct number of frames per roll, but it was probably after 1952, which is when my Darkroom Data-guide notes the 6 exposure rolls were still being used.