Joe, got it, you had me worried for a short while. I remember when the "dual mono" boards came out that several builders were keeping the two channels separate as much as possible, not sharing the floating ground at the RCAs.
Are there any any just perceptible advantages or disadvantages at all when using the plate? Thinking building may be easier with the plate rather than those slip on grounding tab washers. Could wire the RCAs and plate right on the bench and drop it into the chassis.
The plate serves no real purpose other than to be either a convenient or inconvenient assembly aid depending on how you look at it. For a purist dual mono setup, you would separate them but likely you will find the grounds joined at the preamp end connecting to this anyway.