I'm not a 400 owner, ( 3 X 700b's) but always thought that would be a great addition to the 400's.
If it is a genuine WOPL offering, I know it will be a "Touch of Class" !!
......................"Always working on making it better".....
For consideration I've attached the 400 drill guide that was used to mount a front panel toggle switch, AC input fuse, 4 +/- DC fuses, and level controls. Note the date at the bottom. Ignore the address and phone number. My brother, an architect, drew the letterhead.