MOTM

Here is the definitive answer from Uncle Sam.

The *OFFICIAL* spec is EIA-310, Revision D. On page 4,
the center-to-center spec is..........drum roll.................

465.1mm or 18.312 inches, +/- 0.016 inches

Oh brother! 

Now, as Stooge Larry has pointed out, vendors sort of use poetic
license because they figure *slotted* panels. Yes, there even is an
*OFFICIAL* slot spec.

So, I'm (as we used to say at Tandy) screwed, blued and tatooed.

Back to the drawing board...sigh.....

Paul S.