My plan to mount the printed circuit board onto the back of the clock was to use some screwed PCB mounts which were screwed onto double threaded woodscrews fixed into the back of the clock.
I chopped the end of some brass screws and using a thread gauge confirmed that the thread was 3mm x 0.6. I tapped the screw shank and used a hacksaw to cut a screwdriver slot in the top.
I created a pilot hole using a bradawl. Unfortunately when I tried to screw the modified screws into the clock they broke. One I managed to removed but the other need drilling out.
To hide the resultant mess I drilled holes large enough for the standoffs to slot into. I then used a packet of Sugru to hold them in place.