My thanks to Martin Gregori's for his hint.I added a file 75-serialusb.rules to /etc/udev/rules.d containing a single line with
kernel=="ttyUSB*", SYSFS(idVendor)=="067b", SYSFS(idProduct)=="2303", MODE="0666"
and now /dev/ttyUSB0 starts up rw-rw-rw- . Which suffices for my purposes.
