LUGER UNIT MARKINGS

HELLO, I HAVE A LUGER THAT I NEED HELP ON THE UNIT MARKINGS. THE LUGER IS A DOUBLE DATE 1920/1920. AND ON THE FRAME IS THE UNIT MARKINGS. TWO ARE XXX OUT. S.E.I. 2. 23 NEXT IS S.S.2.11. C. 23. AND THE THIRD IS S.E.I.23.....CAN ANYONE BE OF HELP TO ME ? THANK YOU ROBERT HICKEY