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That's really a stunning find!! I say that as one who deeply admires the really old Kirbys, and there aren't all that many of us old farts running around anymore!
One thing I am wondering about it:
I also have a 505, and my bag is definitely original and definitely the herringbone material that the other black-bag-model Kirbys originally had, whereas the bag on the 505 in these photos is corduroy.
So either there was a crossover of material from herringbone to corduroy during the run of the 505 -- not at all impossible -- or it may be a replacement bag, assuming Kirby offered original-color and original-style bags for the 505 but made of the newer corduroy material when the 508 came out with the new gray corduroy and new angular-shaped bag.