This v-shaped notch is a glass forming flaw referred to as a “Notch Defect”. While not easily visible, the notch defect will greatly reduce glass strength in pharmaceutical vials and must be avoided. The picture shows a circumferentially aligned fracture located on the underside of the flange near the barrel junction.  This type of breakage results from an upward directed force applied to the underside outer edge of the flange, generating tensile stress near the base of the flange/barrel junction. Due difficulties in viewing the notch defect, some glass manufacturing companies have started to view this junction under magnification in an optical comparator.