Mine matches:
$ ls -l /dev/ttyACM*
crw-rw---- 1 root dialout 166, 0 Sep 16 09:41 /dev/ttyACM0
What happens when you start the Arduino IDE from the command line? Do you see what I see, or do we have different RXTX versions?
$ ./arduino
Experimental: JNI_OnLoad called.
Stable Library
=========================================
Native lib Version = RXTX-2.1-7
Java lib Version = RXTX-2.1-7
This is very frustrating! I don't know what to do!