We used to sell these but no longer do… the xM1 was a Mifare Classic 1K chip, and our xM1+ was a Chinese knock off of a Mifare Classic 1K chip with a writable sector 0 (changeable NUID)
The problem is that getting the Mifare Classic 1k chip in die format is really difficult, and the sector 0 writable chips were totally unreliable… so we don’t make either anymore.