Rubber pants
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This garment was the predecessor to plastic pants and served the purpose of a diaper cover, replacing the woolen garment. Lacking a fly front, this is a true panty.
They have been almost entirely replaced by plastic or waterproof textile panties as an infants' garment, when such is used over a diaper at all and really only exist in the adult incontinence and 'fetish" market. As an infants' garment they had fallen out of favor in the 1950 but were still the primary adult incontinence protective garment and in that role were called "rubber bloomers"
"Rubber pants" or Rubber Panties" is still the generic term for any pull-on or snap-on incontinence protective garment.