1930s Snag Proof buckle back jeans
Delving a bit deeper into the denim.
From the back side of the denim it is easier to distinguish between the left- and right-hand twill.
The back side of the denim is marked with a red stamp every half yard. As shown in pic 2, the same stamp is used on the left- and right-hand twill denim.
The stamp is almost illegible, but comparing the 10 instances of the stamp and adjusting the contrast, exposure, saturation etc as in pic 5, I am 90% certain that it reads as follows:

6 2/3 OZS.
KINGCOT
REGISTERED

6 2/3 oz is, of course, the old designation of ounce per yard (of 28" fabric), which translates into the current designation of a little over 8 oz per square yard. (Thanks to @denim_seeker for a great conversion table posted recently!)

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