I'm using RPi3 running Raspbian (latest version, recent update)

Tried changing MODE= to be 0777 (in case execute made any difference) - no change

Tried changing SYSTEMS to SYSTEM and error reported was

2020-09-30T15:30:15: [WARNING] Communication with /dev/ttymount @ 9600 failed. Starting Auto Search...
2020-09-30T15:30:15: [ERROR] Failed to connect to port (/dev/ttymount). Error: Port failure Error: Inappropriate ioctl for device. Check if device is connected to this port.

Interestingly a search looking for hints on rules.d under Debian results seem to use SUBSYSTEM (rather than SUBSYSTEMS). Soome to only create the device under /dev with the plural form.
Using as per original udev rules line, then doing udevadm info -n -a /dev/ttymount the initial output is

Udevadm info starts with the device specified by the devpath and then
walks up the chain of parent devices. It prints for every device
found, all possible attributes in the udev rules key format.
A rule to match, can be composed by the attributes of the device
and the attributes from one single parent device.

looking at device '/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.5/1-1.5:1.0/gpiochip3':
KERNEL=="gpiochip3"
SUBSYSTEM=="gpio"
DRIVER==""

looking at parent device '/devices/platform/soc/3f980000.usb/usb1/1-1/1-1.5/1-1.5:1.0':
KERNELS=="1-1.5:1.0"
SUBSYSTEMS=="usb"
DRIVERS=="ftdi_sio"
ATTRS{supports_autosuspend}=="1"
ATTRS{bInterfaceSubClass}=="ff"
ATTRS{interface}=="FT232R USB UART"
ATTRS{bAlternateSetting}==" 0"
ATTRS{bInterfaceClass}=="ff"
ATTRS{authorized}=="1"
ATTRS{bNumEndpoints}=="02"
ATTRS{bInterfaceProtocol}=="ff"
ATTRS{bInterfaceNumber}=="00"

Full output in attached file (in case something further down is relevant), But (recognising bt beginner experience with Linux) that suggests the ttymount does link to the physical device (the FDTI USB serial adapter).

(thanks for your ongoing help)

Ian

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