Looking at the foreground lamp, it looks like there IS a diode installed, i can see the end of it's leg at the right-end, in the V-groove.
Can u show a pic looking down into the wiring area to confirm?

Does this one help? (no diode)....
http://www.pinballspareparts.com.au/index.php?crn=244&rn=1382&action=show_detail

MM.