Inside the bra there's a 2 wide band of fabric along the bottom of each cup (must be that magic inset) see picture. Stein & Co - Hey Viv ! Fabric is
Vintage 1950s Stein & Co White 32B Cotton Rayon Bra Perma-Lift with Magic Inset
Vintage bra 1950s white - Gem
White 1950s Bra
1965 PERMA-LIFT MAGIC Oval Pantie Girdle & Bra photo nude color vintage print Ad $8.29 - PicClick
1955 VINTAGE PERMA-LIFT Bra Rounded Look with Magic Insets Look Enchanting Ad $5.99 - PicClick
New Old Stock Bra
Permalift
Vintage 1950s white rayon - Gem
50s Bra