Thanks for your persistence; I've checked your points above and the problem remains.
Here's the rule file listed in /etc/udev/ruled.d/* :
-rw-r--r-- 1 root root 371 Mar 21 13:03 99-USB_ISP_R3.rules
Should it be named differently, I wonder?
My username is in the group plugdev.
I'm using Ubuntu 14.04 LTS.
Thanks again.