This is also a 504ZXX, a zippered version of the 501xx youth model, with the offset belt loop. And it was probably made in 1963 or 1964, with about 70% color residue. At the time of purchase, there was a large light brown stain on the back, mainly on the thighs, and from the smell, it looked like glue. But after purchase, the stain was completely removed after being soaked in lukewarm water.
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