The flexM1 v2 is a Mifare "Classic" S50 1k chip emulator with writable Sector 0, also known as the "manufacturer block" (includes the 4 byte NUID "serial number").
13.56MHz Mifare S50 1k emulator chip
4 byte NUID and all of sector 0 is writable
10.25mm x 13.5mm x 0.65mm flexible biopolymer package