Vintage Bogner Classic White Half Slip w/ Side Button Closure and Side Slits

This unusual slip may have been part of a dress or skirt? Bogner is still around, but can't find any Bogner slips, mostly skirts and outerwear! Unusual waist. Elastic on one side, button closure on one side of slip. Also wide side slits 14 1/2" deep each Waist 29" + Length about 30" In EUC, like new without flaws.